The Unity of Consciousness (2025)

1. History

The unity of consciousness was a main concern of most philosophers inwhat is often called the ‘classical modern era’ (roughly,1600 to 1900), including Descartes, Leibniz, Kant, Hume (in a way; seebelow), Reid, Brentano, and James.

Consider a classical argument of Descartes’ for mind-bodydualism. It starts like this:

When I consider the mind, that is to say, myself inasmuch as I am onlya thinking thing, I cannot distinguish in myself any parts, butapprehend myself to be clearly one and entire. [Descartes 1641: 196]

Descartes then asserts that if the mind is not made of parts, itcannot be made of matter because anything material has parts. He addsthat this by itself would be enough to prove dualism, had he notalready proven it elsewhere. Notice where it is that I cannotdistinguish any parts. It is in “myself inasmuch as I am only athinking thing” (ibid.); that is, in myself as awhole—which requires unified consciousness of myself as a whole.The claim is that this subject, the target of this unifiedconsciousness, is not a composite of parts.

In Kant (1781/7), the notion that consciousness is unified is centralto his ‘transcendental deduction of the categories’ (seethe entry onKant’s view of the mind and consciousness of selffor a fuller treatment of Kant). There Kant claims that in order totie various objects of experience together into a single unifiedconscious experience of the world, we must be able to apply certainconcepts to the items in question. In particular, we have to applyconcepts from each of four fundamental categories of concept:quantitative, qualitative, relational, and what he called‘modal’ concepts. Kant’s attempt to link the unityof consciousness to the structure of knowledge continues to capturethe imaginations of philosophers: Arguments of this form can be foundin P. F. Strawson (1966), Cassam (1996), Hurley (1994, 1998) andRevonsuo (2003), and are examined critically inSection 7.3and in Brook (2005).

Kant was familiar with arguments of the kind that we just sawDescartes mount (chiefly from similar reasoning in Leibniz andMendelssohn) but he was not impressed. For Kant, that consciousness isunified tells us nothing about what sorts of entity minds are,including whether or not they are made out of matter (1781, chapter onthe Paralogisms of Pure Reason). He argues that the achievement ofunified consciousness by a system of components acting together wouldbe no more or less mysterious than its being achieved by somethingthat is simple, i.e., has no components (1781: A352).

Leibniz, Hume, Reid, Brentano, and James held a variety of positionson unity. Briefly, for Leibniz (see the entry onLeibniz’s philosophy of mind)unified consciousness and the noncompositeness, the indivisibilitythat he took to be required for it seem to have served as his model ofa monad, the building block of all reality. With Hume (1739), thingsare more complicated. It should have followed from his atomism thatthere is no unified consciousness, just “a bundle of differentperceptions” ([1739] 1962: 252). Yet, in a famous appendix, hesays that there is something he cannot render consistent with hisatomism (p. 636). He never tells us what it is but it may have beenthat consciousness strongly appears to be more than a bundle ofindependent ‘perceptions’. Reid (1785), almost an exactcontemporary of Kant’s, made extensive use of the unity ofconsciousness, among other things to run Descartes’ argumentfrom unity to indivisibility the other way around. Brentano (1874)argued that all the conscious states of a person at a time will andperhaps must be unified with one another. (He combined this view withanother strong thesis, that all mental states are conscious.) Finally,late in the 19^th century James developed a detailedtreatment of synchronic (or ‘at a time’) unity ofconsciousness. We will discuss his view later (see also entries onDavid Hume,Thomas Reid,Franz Brentano,andWilliam James).

Early in the 20^th century, the unity of consciousnessalmost disappeared from the research agenda. Logical atomism inphilosophy and behaviourism in psychology had little to say about thenotion. Logical atomism focussed on the atomic elements of cognition(sense data, simple propositional judgments, protocol sentences,etc.), rather than on how these elements are tied together to form amind. Behaviourism urged that we focus on behaviour, the mind beingeither a myth or at least something that we cannot and do not need tostudy in a science of the human person.

One partial exception to this pattern of neglect was Gestaltpsychology. Indeed, Gestalt psychology was sufficiently influential inits time that some positivists tried to make their systems compatiblewith it (Smith 1994: 23). For instance, Carnap chose to avoid anycommitment to atoms of experience as the elements of his system,opting instead for ‘total experiences’. As we will see, anotion similar to his concept of irreducible experiential wholes canbe fruitful(Section 7.4).However, Carnap seems to have had something rather different in mindfrom what philosophers now have in mind when they speak of the unityof consciousness. Gestalt unity is a unity in a structure of which oneis conscious, where the way in which each part appears is derived fromthe structure of the whole (Tye 2003: 11–5; Bayne & Chalmers2003: 27). This is distinct from unity in one’sconsciousness of objects, objects that need not themselvesexhibit the qualities of gestalt structures.

After decades of neglect, the last third of the 20^thcentury saw a resurgence of interest in unified consciousness amonganalytic philosophers. It began with influential commentaries on Kantin the 1960s (Strawson 1966; Bennett 1966, see also his 1974), as wellas discussions by Nagel (1971) and Parfit (1971, 1984). More recently,quite a number of philosophers and a few psychologists have written onthe subject, including Marks (1981), Trevarthen (1984), Lockwood(1989, 1994), Hill (1991), Brook (1994), Marcel (1994), Hurley (1994,1998), Shoemaker (1996, 2003), O’Brien and Opie (1998). The firstdecades of the 21^st century have seen a lot of importantnew work on the subject: Dainton (2000), Stevenson (2000), Bayne andChalmers (2003), Hurley (2003), Kennett and Matthews (2003), Rosenthal(2003), Radden (2003),Tye (2003), Zeki (2003), Nikolinakos (2004), Brook and Raymont (2006),LaRock (2007), and Bayne (2008, 2010, the latter a book-length study).Cleeremans (2003) is an excellent collection containing papers byphilosophers such as Bayne, Chalmers, Hurley, Shoemaker, Cotterill,and Thompson and psychologists such as Triesman, Humphreys, Engel,Diennes, Perner, and Varela. Blackmore (2004, especially ch. 17) is agood, scientifically-oriented introduction. The section on the unityof consciousness in PhilPapers has 391 entries (as of April 2017), thevast majority from the last twenty years (seeOther Internet Resources).

2. Characterizations and Taxonomies

2.1 Characterizations

What characterizes the unity of consciousness? Note that this questioncan be asked and answered whether or not there is any suchthing as unified consciousness. Indeed, we need to know what the unityof consciousness is like even to address the question of itsexistence. (For ease of exposition, we will write as though there isunified consciousness, even though the question really remains openuntil the next section.)

That said, it should also be noted that it is difficult to say muchabout the unity of consciousness that is both non-question-begging andmore than a thinly disguised synonym, a point that Dainton (2000)emphasizes. Even as great a theorist of the subject as Immanuel Kantthrew up his hands. He observed that this unity is “not thecategory of unity” (B131), that is to say, is not just a matterof being numerically one—and said no more.

Underlying the various attempts to identify what is characteristicabout the unity of consciousness are two opposing views of thestructure of a unified conscious experience. On what we will call theexperiential parts view (EP), a unified conscious experience is acomposite of other experiences. The no experiential parts view (NEP)denies this, asserting that while a unified conscious experience willhave a complex object or content, it has no experiential parts. Onthis view, when the objects of particular experiences get incorporatedinto a ‘bigger’ unified experience, the new experiencereplaces the particular experiences rather than containing them asparts. It would be premature to discuss the two views here (seeSections7.1and7.2)but we need to know about them to understand the attempts that havebeen made to characterize unity. The first two ways of characterizingthe unity of consciousness that we will examine are within theexperiential parts approach.

2.1.1 Subsumption

One increasingly prominent attempt to characterize the unity ofconsciousness holds that in unified consciousness, particularexperiences are subsumed in a more complex experience. For example,Bayne and Chalmers (2003) say that when particular experiences areunified, they are “aspects of a single encompassing state ofconsciousness” (see also Bayne 2010: 20, 31). More precisely,two experiences are what they call ‘subsumptively unified’“when they are both subsumed by a single state ofconsciousness” (2010: 27). This yields a distinctivephenomenology. Two subsumptively unified states will have what theycall a conjoint phenomenology: a phenomenology of having both statesat once that subsumes the phenomenology of the individual states:“there is something it is like for the subject to be in [twoconscious] states simultaneously” (2010: 32).

One feature of subsumption is that it requires there to beexperiential parts. Thus, those who favour NEP or even wish acharacterization of unified consciousness to be neutral on this issuewill look for a different account.

2.1.2 Co-Consciousness

A second attempt to characterize unified consciousness claims that arelation among local conscious states is the crucial element, arelation usually called co-consciousness. As Jamesput it, in synchronic unified consciousness, we are co-conscious ofA, B, and C (1909: 221). Others who centre theiranalysis on the notion include Parfit (1984) and Hurley (1998). Thesetheorists seldom try to define the term‘co-consciousness’. They treat the notion as beingintuitively clear and let it function as a primitive in theiranalysis.

Like subsumption, most versions of co-consciousness requireexperiential parts (James, who accepted NEP and thus had an unusualconception of co-consciousness, is an exception). In addition to aproblem of lack of neutrality, this requirement faces the problem thatsome forms of unified consciousness do not seem to involvemultiplicity of items, unified consciousness of self for example. Ifthere is no multiplicity of items, there is nothing to enter into a‘co’-relationship.

A number of theorists combine the two approaches. Many of the peoplewho use the term ‘co-consciousness’ in fact seem to havesubsumption in mind as the underlying notion. Dainton, for example,embraces the language of co-consciousness and relates the notionto“being experienced together” [subsumption] (2000: 236)but urges in the end that co-consciousness is a better primitive thansubsumption (Dainton 2005: 258–259). Or Lockwood (1989: 88):co-consciousness is “the relation in which two experiencesstand, when there is an experience of which they are bothparts”. Similarly Shoemaker (2003: 65): “The experiencesare co-conscious … by virtue of the fact that they arecomponents of a single state of consciousness … ”.

2.1.3 Joint Consciousness

There are at least two approaches to what characterizes unifiedconsciousness that are compatible with NEP. One we find in Tye. Inunified conscious states, the things that we experience are“experienced together”, “enter into the samephenomenal content” (2003: 36, hisemphasis)—which phenomenal content could be the content of asingle non-composite experience.

Another has been advanced by Brook and Raymont (Brook 1994: 38; Brook2000; Brook & Raymont 2006). The key idea is what they call jointconsciousness. Joint consciousness is present when the followingholds: If an experience that one is having provides consciousness ofany item, then it provides consciousness of other items and of atleast some of the items as a group. Likewise for consciousness of actsof experiencing.

This notion tries to capture what is distinctive about unifiedconsciousness in a way that is neutral with respect to the EP/NEPdebate. The notion is related to the phenomenal side of theBayne/Chalmers notion of subsumption, there being something it is liketo be in two conscious states simultaneously (Bayne & Chalmers2003: 32), but in a way that is free of the non-neutral notion ofsubsumption. It is not clear whether joint consciousness is analternative to Tye’s notion of same phenomenal content or anattempt to say something about what yields such content.

2.2 Taxonomies

Most contemporary theorists agree that unified consciousness can takea number of forms. Many schemes for dividing it up exist in theliterature. Tye (2003: 11–5), for example, distinguishes objectunity, neurophysiological unity, spatial unity, subject unity,introspective unity, and, finally, phenomenal unity. The latter is thenotion that he explicates in terms of contents being experiencedtogether, entering into the same phenomenal content, and is the notionon which he focuses.

Similarly, Bayne and Chalmers (2003: 24–7) distinguish objectualunity (a matter of two conscious experiences of one object, sodifferent from Tye’s object unity), spatial unity, subjectunity, and subsumptive unity, the last a matter of two or moreconscious states becoming aspects of a single conscious state. Thenwithin subsumptive unity, they distinguish between access unity andphenomenal unity. We just examined their definition of the latter. Asis true of Tye, it is what mainly interests them. Two conscious statesare phenomenally unified “if there is something it is like forthe subject to be in both conscious states simultaneously”.

Kant and philosophers in the Kantian tradition break phenomenal unitydown. The division usually follows the traditional division ofexperience into subject, representation, and object or content,assigning to each its own form of unified consciousness. Thus therewill be unified consciousness of individual objects, of multiples ofobjects, of acts of experiencing, and of oneself as the subject ofsuch experiencing. (A fifth form can be distinguished, too, as we willsee.) Few contemporary theorists break phenomenal unity down at all,so this division is of some interest.

The first three forms of unified consciousness in the Kantiantradition can be expressed in terms of the notion of jointconsciousness just introduced. First, unified consciousness ofindividual objects. This is Tye’s object unity; Bayne andChalmers’ objectual unity is at least a related notion. Theprocess at work here is now commonly called binding (Hardcastle 1998;Revonsuo 1999). Binding is the process of tying various features of avisual scene such as colour, shape, edges, and contours, featuresdetected in various places in the visual cortex, together into anexperience of a unified, three-dimensional object. Binding may benecessary for consciousness of individual objects but it does not seemto be sufficient. We must, it seems, also be jointly conscious of thevarious elements to have unified consciousness of an object.

Next, unified consciousness of contents. In unifiedconsciousness of contents, if an experience that one is havingprovides consciousness of any object or content, then it providesconsciousness of other objects or contents and of at least some of theitems as a group. Here ‘object’ and ‘item’cover objects, properties, events—anything of which one can beconscious. (We speak of experiences rather than representations indeference to those who doubt that we experience in representations, orneed do so: we wish to be neutral on this issue.)

This distinction between unified consciousness of individual objectsand of multiples of objects corresponds closely to two kinds ofsynthesis distinguished by Kant (1781/7, First Division, Book 1,Chapter 2). Kant distinguishes between the kind of acts of synthesisneeded to attain consciousness of individual objects and the kind ofacts of synthesis needed to attain consciousness of a number ofobjects at the same time as a single array of objects experienced by asingle subject (Brook 1994: 123). He builds his argument for necessarycausal connectedness on the latter.

Unified consciousness of contents appears to be central to our kind ofconsciousness. For example, suppose that one is conscious ofone’s computer and also of the car sitting in one’sdriveway. If consciousness of these two items were not unified, animportant, indeed probably the most important, way of comparing themwould not be available. One could not answer questions such as, Is thecar the same colour as the WordPerfect icon?, or even, Is the car tothe left or to the right of the computer? That is what unifiedconsciousness does for us: it allows us to make such comparisons.Since relating item to item in this and related ways is fundamental toour kind of cognition, unified consciousness is fundamental to ourkind of cognition. As we will see inSection 4.1,there are disorders of consciousness in which this ability to compareseems to be lost. These disorders leave people with a massivecognitive impairment.

Most theorists outside of philosophy and many within accept that thereis a second form of conscious unity related to unified consciousnessof contents, namely, unified consciousness of acts ofexperiencing. It is present when, for the current acts ofexperiencing that one is doing, consciousness of one act ofexperiencing (consciousness of how one is experiencing something, forexample seeing it, imagining it, …) provides consciousness of otheracts of experiencing. (This explication is structured to be neutral asto whether unified conscious states include a multiplicity ofconscious states. We speak of ‘consciousness of acts ofexperiencing’ rather than ‘consciousness of anexperience’ for the same reason.)

Not all theorists accept that this second form of unifiedconsciousness exists. Those who promote the so-called transparencythesis, the claim that we are not directly conscious of our ownexperiencings, deny that we have any such form of consciousness(Dretske 1995; Tye 2003). Tye, for example, says that when we hearsomething, we are not conscious of the auditory experience, just whatit represents. If one tries to be conscious of the experience, at bestone is aware only of “the auditory qualities that the experiencerepresents” (2003: 33).

Many theorists have also had a fourth thing in mind when they speak ofthe unity of consciousness, namely, unified consciousness ofoneself, the thing that has the experiences. Here, one is orcertainly seems to be (see the discussion of Rosenthal inSection 3.1)conscious of oneself not just as subject but, in Kant’s words(A350), as the ‘single common subject’ of many or all theaspects of the unified experience that one is now having and of anumber of similar experiences past and, in anticipation, still tocome. (Mutatis mutandis, the same holds for the single commonagent of various bits of deliberation and action.) One has unifiedconsciousness of self when one is conscious of oneself as the singlecommon subject of experiences of many items in many acts ofexperiencing.

The unified consciousness here seems not to be a matter of jointconsciousness. When one is conscious of oneself as the common subjectof one’s current unified acts of experiencing and of unifiedacts of experiencing past and to come, one is not conscious of anumber of objects, nor a number of acts of experiencing either.(Indeed, if Kant is right, when one is conscious of oneself assubject, one need not be conscious of oneself as an object atall (A382, A402, B429).) However, something similar might be at work.One seems to be conscious of one and the same thing as one and thesame thing, namely, oneself, via a number of acts ofexperiencing.

Unified consciousness of self has been argued to have some veryspecial properties, for example that the reference to oneself asoneself by which one achieves consciousness of oneself as subject mustbe indexical and cannot make use of ‘identification’(Castañeda 1966; Shoemaker 1968; Perry 1979). Generalizing thelatter notion, it has been claimed that reference to self does notproceed by way of attribution of properties or features to oneself atall (Brook 2001). One argument for this view is that one is or couldbe conscious of oneself in the same way as the subject of each andevery one of one’s conscious experiences. If so, one would notbe conscious of oneself as one kind of thing rather than another. AsBennett (1974: 80) once put it, consciousness of self would not be‘experience-dividing’—statements expressing it wouldhave “no direct implications of the form ‘I shallexperience C rather than D’”. And if that isso, one’s consciousness of self might not be gained via beingconscious of features of oneself at all

Some theorists especially in the empirical literature hold that afifth form of phenomenal experience should attract the label,‘unified consciousness’. We might call it unity offocal attention. It differs from the other forms of unifiedconsciousness that we have delineated. In the others, consciousnessranges over either many experienced items (unified consciousness ofcontents), experiencings of many objects (unified consciousness ofexperiencing), or multiple acts of access to oneself as subject ofmany experiencings (unified consciousness of self). Unity of focuspicks out something within these unified ‘fields’. WilhelmWundt captured what we have in mind in his distinction between thefield of consciousness (Blickfeld) and the focus ofconsciousness (Blickpunkt; Wundt [1874] 1893: Vol. II, 67).The consciousness of a single item on which one is focussing isunified because one is conscious of many aspects of the itemin one state or act of consciousness (especially relational aspects,e.g., any dangers it poses, how it relates to one’s goals,etc.), and of many different considerations with respect to that item(one’s goals, how well one is achieving them with respect tothis object, etc.), in the same state or act of consciousness. Inunified focal attention, one integrates a number of cognitiveabilities and applies them to an object. Bayne andChalmers’ objectual unity is a related notion (2003:24–25). Note that, if there are forms of unified consciousnessdifferent from focal attention, then, contrary to Posner (1994) andothers, attention is not a component of all forms of consciousness(Hardcastle 1997).

All five of the forms of phenomenal unity can, to one degree oranother, be attributed to Kant. The first four are clearly Kantian butthere is a connection to him even with respect to the fifth. Hedidn’t speak of attention very often but he did speak of it.See, for example, B156n.

Since Hume ([1739] 1962: 252) and Rosenthal (2003) deny that we havesuch unified consciousness of self and Dennett (1991, 1992) says atminimum that we have much less of it than we think, it is perhapspertinent to say again that in this section we are merely trying tosay what the various forms of unity would be like if they exist.Whether they do exist is the topic ofSection 3.

2.3 Other Forms of Mental Unity

We will close this section by noting that the forms of unifiedconsciousness distinguished above are not the only kinds of mentalunity. Earlier we mentioned Gestalt unity. There is alsounity in the exercise of our cognitive capacities, unity that consistsof integration of motivating factors, perceptions, beliefs, etc., andthere is unity in the outputs, unity that consists of integration ofbehaviour.

Human beings bring a strikingly wide range of factors to bear on acognitive task such as seeking to characterize something or trying toreach a decision about what to do about something. For example, we canbring to bear: what we want; what we believe; our attitudes to self,situation, and context; input from each of our various senses;information about the situation, other people, others’ beliefs,desires, attitudes, etc.; the resources of however many languages wehave available to us; various kinds of memory; bodily sensations;diverse problem-solving skills; and so on. Not only can we bring allthese elements to bear, we can integrate them in a way that is highlystructured and ingeniously appropriate to our goals and thesituation(s) before us. This form of mental unity could appropriatelybe called unity of cognition.

It is plausible to hold that unity of cognition is required for unityof focal attention. However, there is at least some measure of unifiedcognition in many situations of which we are not conscious, as isattested by our ability to balance, control our posture, maneuveraround obstacles while our consciousness is entirely absorbed withsomething else, and so on.

At the other end of the cognitive process, we find an equallyinteresting form of unity, what we might call unity ofbehaviour: our ability to coordinate our limbs, eyes, bodilyattitude, etc. The precision and complexity of the behaviouralcoordination we can achieve would be difficult to exaggerate. Think ofa concert pianist performing a complicated work. However, thiscapacity to unify behaviour, though doubtless a product of unifiedconsciousness, does not figure in what unified consciousness is.

3. Is Consciousness Unified?

Now that we know what we are talking about when we talk about unifiedconsciousness, the next question to ask is: Does it exist?Does consciousness have the properties that it would need tohave to be unified? If this division of questions looks peculiar,notice that it can apply to Santa Claus, too. We can develop anaccount of what Santa Claus would be like without committing ourselveson the question of whether such a being exists. However, the divisionmay look peculiar in another way: How could anyone deny thatconsciousness is unified? That it is seems just obvious. In fact,there has been a good deal of skepticism on the matter.

Some will urge that before we ask whether unified consciousnessexists, we should first ask, Does consciousness exist? Therenow seems to be a wide consensus that the answer to this question is,Yes, there is in us something appropriately called‘consciousness’. Even those who hold that thelong-standing idea thatintentionality (see entry)is a matter of attitudes to propositions is false and ripe forelimination, Paul and Patricia Churchland for example, allow thatconsciousness exists, though they urge that the concept be trimmed abit (see, for example, Patricia Churchland 1983). Some writers havetaken Dennett (1991) to deny that consciousness exists, eitherdirectly or by implication. He himself has said repeatedly thatconsciousness is real, however (1995: 135, 146 are twoexamples). A few writers, Wilkes (1984) and Rey (1988) for example,have espoused true eliminativist about consciousness but they are atiny minority.

3.1 Skepticism about Unity

Many philosophers have been sceptical about whether consciousness isunified. It is possible to be sceptical about whether allconsciousness is unified, whether as many conscious states are unifiedas we might think, and, the strongest form of scepticism here, whetherthere is any unity of consciousness. Let us examine the three inreverse order.

Hume ([1739] 1962: 252) seems to have been an example of the strongestform of scepticism. He famously doubted, or tried to doubt, that wehave unified consciousness even of the self, let alone of one’sconscious states. Among recent writers, perhaps the most scepticalabout consciousness being unified is Rosenthal. Rosenthal holds thatall we have is a “sense of the unity ofconsciousness” (1986: 344, emphasis added). Why merely a sense?On his view,

Mental states are conscious, when they are, in virtue of their beingaccompanied by HOTs [higher-order thoughts] and each HOT representsits target as belonging to the individual who also thinks the HOT inquestion. (Rosenthal 2002: 15)

Across a range of such self-ascriptions, one develops a sense of beingtheir common subject. However, this sense could be wrong. Theexperiences thus ascribed, says Rosenthal, could be supported by orlocated in a diversity of subjects. It is because of this possibilitythat Rosenthal asserts that all we have is a sense of consciousnessbeing unified.

Even when theorists such as Hume and Rosenthal deny that consciousnessis unified, sometimes unity of some sort still seems to be at work intheir models. Rosenthal says, for example, “A mental state isconscious just in case it is accompanied by a … thought to theeffect that one is in the state in question” (2003:325, our emphasis). If so, in addition to a HOT being about anotherpsychological state, it is also about oneself, the thing that has thestate (‘that oneis in the state inquestion’). Now, this consciousness of oneself is notconsciousness of any old object, it is consciousness of oneself,oneself as the bearer of conscious states. But this is consciousnessof oneself as, to use Kant’s phrase, the single common subjectof one’s experience (1781/7, A350). If so, Rosenthal allows thatone kind of unified consciousness exists despite himself.

More recently, Nagel (1971), Davidson (1980), Dennett (1991, 1992),O’Brien and Opie (1998), and Rosenthal (2003) have all urged inone way or another that the mind’s unity has been overstated.The point these people make is not just that the mind works mostly outof the sight and often out of the control of consciousness. Virtuallyeveryone agrees on that and the point would in no way tell againstthere being a real unity of some kind in the part that does enterconsciousness. Rather, they maintain that not even all consciousstates are unified with other conscious states.

Note that the claim here is not just that we are conscious of less ofthe contents of our mind than we think, as Freud and thepsychoanalytic tradition argue. The claim is that our consciousness ofeven many states of which we are conscious is not, or not fully,unified. That we act against what we clearly know to be our own mostdesired course of action or do things while telling ourselves that wemust avoid doing them are advanced as reasons for holding the view butthese are not obvious examples of lack of unity. A change blindnessexperiment might offer a more substantial example.

In this experiment, a subject sits in front of a computer screenwearing an eye tracker visor. There is a paragraph of text on thescreen. The subject is asked to read the text. When subject’seyes are focused on a particular word or phrase, the bit of text is asit should be. However, everything around the word or phrase atfixation, everything above, below and on both sides, beyond about 5degrees of arc is chaotic: gibberish, out of order words, shapesvaguely like words but obviously not words, and so on. Each time thesubject’s eyes shift to a new bit of text, it pops into correctform just before the eyes get to it and the word or phrase just leftgoes strange. What is remarkable about this experiment is that, whileto observers all appears chaotic except for a succession of words orphrases—they are marching to a different saccadicdrummer—the subject has no consciousness that anything in theparagraph is ever out of the ordinary. If the subject is one of thosepeople who automatically reads any text in front of her, often she hasno idea that the experiment has even started!

What this experiment shows, Dennett (1991: 361–362) thinks, isthat we take only a tiny, central bit of the scene in front of usfully into unified consciousness. Yet subjects are conscious of therest of the screen in some ways. They are aware of movement, forexample. If so, this experiment would be an example of consciousstates that are not fully taken up into unified consciousness. Wewould have unified consciousness of fewer of our conscious states thanhas been thought.

Note that the argument just examined is not an argument that unifiedconsciousness does not exist. Even if we have conscious states thatare not unified or fully unified in consciousness, the most that thatcould force us to do would be to shrink the range of states over whichconsciousness is unified or fully unified. From the fact that not allof one’s conscious contents are unified, it does not follow thatnone is.

Indeed, those who hold that the extent to which consciousness isunified has been overstated owe us an account of what hasbeen overstated. When theorists claim that a some conscious states arenot in unified consciousness, we should ask: Not unified with what?One plausible answer would be: The unified conscious mind. Here is oneway to view the matter. Once upon a time, some theorists held that allconscious states are unified and indeed that all mental states areconscious. As we saw, Brentano is an example. (We will return to thisview in the next Section.) The main difference between thispre-twentieth century vision of unified consciousness as ranging overeverything in the mind and view we just examined is that on the latterview, less of consciousness is unified than the earlier view held.

Dennett is interesting in this regard. As we saw, he can plausibly beread as rejecting the traditional picture of unified consciousness.Yet he can still invoke unity. He says, “What is it like to bean ant colony? Nothing, I submit … What is it like to be abrace of oxen? Nothing (even if it is like something to be a singleox)” (Dennett 2005). Why is the answer nothing? In such cases,“there is no functional unity …—no unity todistinguish an I from a we” (Dennett 2005).

To sum up the discussion of scepticism about unified consciousness sofar, the argument that the unity of consciousness is real, indeed is acentral feature of our kind of mind, seems to be strong.

3.2 Consciousness Must be Unified

That said, there are theorists who maintain not only that someconscious states in a subject are unified but that all consciousstates must be unified. Bayne and Chalmers (2003: 24; Bayne2010: ch. 1.4) call this the unity thesis: necessarily, anyset of conscious states of a subject at a time is unified. As we saw,Brentano probably held this view. Hill (1991) does, too. Scepticismabout this view would be weaker than either of the two kinds ofscepticism about unified consciousness that we just examined.

Bayne and Chalmers express sympathy for the thesis on the groundsthat,

It is difficult or impossible to imagine a subject having twophenomenal states simultaneously, without there being a conjointphenomenology for both states. (2003: 37)

Merely having phenomenal states might seem too little butBayne and Chalmers are talking about phenomenal states where, forthem, to have the state is for the state to be like something. If werecast to make this element explicit, we get a claim of some realintuitive appeal: If A is like something to S andB is like something to S, it must be the case that thecombination, A and B, is like something to S.Interestingly, Kant seems to have believed something similar:“[Experiences] can represent something to me only insofar asthey belong with all others to one consciousness” (A116).A and B having conjoint phenomenology is exactly whatunity consists in, according to Bayne and Chalmers. Put this way, theunity thesis has some real appeal.

Are there reasons to be sceptical of the unity thesis, presumptivecounter-examples say? Spelled out as we have spelled it out, we do notknow of any. Bayne and Chalmers consider brain bisection cases to beputative counter-examples because, on some concepts of the subject ofexperience, we can think of there still being one subject in thesecases even though not all the conscious states are unified. There areat least three ways to respond. The simplest is just to deny thatthere is one subject, at least for the period of the split. A secondwould be to note that, however one counts subjects during the periodof the split, there is evidence that many conscious experiences inthat body are not like anything to some subject. If so, the apparentlack of conjoint consciousness of them will not be a problem.A third (advocated by Bayne & Chalmers 2003: 38–9; Bayne2010: ch. 9) would be to urge that while there is clearly a breach inthe unity of access consciousness (access to information for purposesof belief formation, behavioural control, and so on) during the periodof the split, phenomenal unity may still extend across all theconscious experience. We will discuss the third response in moredetail inSection 4.2below.

The unity thesis is a very strong thesis. Theorists could hold boththat consciousness is unified and that this unity is important and yetdeny that the unity thesis is true.

4. Disorders of Unified Consciousness

One of the most interesting ways to study psychological phenomena isto see what happens when they take an abnormal form or break down.Phenomena that look simple and seamless when functioning smoothlyoften reveal all sorts of structure when they begin to malfunction.The unity of consciousness can be damaged and/or distorted in bothnaturally-occurring and experimental situations. What can we learnfrom these cases?

As a rough grouping tool, we will treat unified consciousness asbreaking down in ways that fit a two-by-two matrix. The cases we willconsider can be grouped according to whether unified consciousnessappears to continue but in an unusual form, one instance of itsplitting into two for example, and situations in which it appears todeteriorate more severely, to the point where it may even be said tohave been destroyed. And we will find examples of both kinds thatoccur at a time and across time. Indeed, some of the disorders that wewill examine are both at a time and across time.

At a time, situations of disorder in which unified consciousness isretained may take a number of forms. There are cases whereconsciousness seems to split within one brain and body. The muchdiscussed commissurotomies (brain bisection operations) are the bestknown example. (Across time, the closest analogue may be dissociativeidentity disorder, about which more shortly.) There may be cases inwhich a single occurrence of unified consciousness spans two bodies;some have said this about some mirror twins. And there are cases inwhich the array of phenomena over which unified consciousness rangesbecomes strangely circumscribed: hemi-neglect and anosognosia are twoexamples. (We will describe all these phenomena in more detail below.)What is interesting for our purposes is that in all these kinds ofcases, those in which unified conscious seems to split, those (if any)in which it seems to span two bodies, and those in which its rangeseems to shrink, the unity itself seem to be intact. It has not beendestroyed or even damaged. It is just housed in an unusual way.

The cases just introduced contrast with situations in which we havejust one instance of consciousness of some kind ranging over the usualphenomena or some of them, but where the unity, to dramatize a bit,appears to have shattered, not split or expanded or shrunk. In thesecases, the unity of consciousness is not just unusually housed. It hasbeen seriously compromised.

4.1 Some Disorders

4.1.1 Brain Bisection Operations

No medical procedure to do with consciousness has received as muchphilosophical attention in recent times as commissurotomies, morecommonly known as brain bisection operations. Nagel (1971) was perhapsthe first philosopher to write on them; his paper continues to beinfluential. Since then, Puccetti (1973, 1981), Marks (1981), Hirsch(1991), Lockwood (1989), Hurley (1998), Bayne (2008, 2010), Schechter(2010) and many, many other philosophers have written on theseoperations. Indeed, the strange results of these operations in certaincontrolled conditions was one of the things that brought the unity ofconsciousness back onto the cognitive research agenda.

In these operations, the corpus callosum is cut. The corpus callosumis a large strand of about 200,000,000 neurons running from onehemisphere to the other. When present, it is the chief channel ofcommunication between the hemispheres. These operations, done mainlyin the 1960s but recently reintroduced in a somewhat modified form,are a last-ditch effort to control certain kinds of severe epilepsy bystopping the spread of seizures from one lobe of the cerebral cortexto the other. For details, see Sperry (1984), Zaidel et al. (1993), orGazzaniga (2000).

In normal life, patients show little effect of the operation. Inparticular, their consciousness of their world and themselves appearsto remain as unified as it was prior to the operation. How this can behas puzzled a lot of people (Hurley 1998). Even more interesting forour purposes, however, is that, under certain laboratory conditions,these patients seem to behave as though two ‘centres ofconsciousness’ have been created in them. The original unityseems to be gone and two centres of unified consciousness seem to havereplaced it, each associated with one of the two cerebralhemispheres.

Here are a couple of examples of the kinds of behaviour that promptthat assessment. The human retina is split vertically in such a waythat the left half of each retina is primarily hooked up to the lefthemisphere of the brain and the right half of each retina is primarilyhooked up to the right hemisphere of the brain. Now suppose that weflash the word TAXABLE on a screen in front of a brain bisectedpatient in such a way that the letters TAX hit the left side of theretina, the letters ABLE the right side, and we put measures in placeto ensure that the information hitting each half of the retina goesonly to one lobe and is not fed to the other. If such a patient isasked what word is being shown, the mouth, controlled usually by theleft hemisphere, will say TAX while the hand controlled by thehemisphere that does not control the mouth (usually the left hand andthe right hemisphere) will write ABLE. Or, if the hemisphere thatcontrols a hand (usually the left hand) but not speech is asked to doarithmetic in a way that does not penetrate to the hemisphere thatcontrols speech and the hands are shielded from the eyes, the mouthwill insist that it is not doing arithmetic, has not even thought ofarithmetic today, and so on—while the appropriate hand is busilydoing arithmetic!

Recently, this standard assessment has been challenged. In hisimportant 2010 book and other publications, Tim Bayne suggests thatthere are not two centres of consciousness. There is just one. Whatexplains the appearance of duality is that this single centre ofconsciousness switches in the material of which it is consciousnessfrom one hemisphere to the other. He calls this the switch model(2010: chs 8.4 and 9.5). To so much as get this approach off theground, it is crucial that that there be little or no evidence of acentre of consciousness that is conscious of, say, A, B,and C but not D, E, and F and at the sametime of a centre of consciousness that is conscious of D,E, and F but not A, B, C. What wejust called the standard assessment starts from a belief that there islots of such evidence. Bayne disagrees. We will return toBayne’s view inSection 4.2.

Because brain bisection operations have attracted so much attentionoutside of psychiatry and neurology, we have included references tosome of the more important writings. We will not do so for the rest ofthe disorders we will introduce. For fuller accounts and references,consult a general textbook of psychiatry. Here we are interested inthem only for the vicissitudes of unified consciousness that theydisplay, or might be thought to display.

4.1.2 Mirror Twins

In addition to cases in which one body may have two centres of unifiedconsciousness, there are cases in which one centre of unifiedconsciousness may span two bodies. Mirror twins sometimes have such anappearance. Mirror twins are biologically identical twins where eachbody mirrors the other. They dress exactly the same, finish eachother’s sentences, do tasks together whenever possible (if anegg is being cooked, both right hands will hold the handle of thefrying pan), and so on. Brain bisection cases are putative examples ofthe number of centres of consciousness not lining up with the numberof brains and bodies from one direction. Mirror twins might be anexample from the opposite direction. In connection with unifiedconsciousness, this possibility is interesting.

The best-known case is the case of Greta and Freda Chaplin, who cameto light in the U.K. in the 1970s. Two bodies were involved but thebodies acted in ways that would have been compatible, at least, with asingle instance of unified consciousness spanning them. Each bodycould finish complicated and unpredictable sentences started by theother. The two did everything they could together. When separated bymore than a few metres, they complained bitterly, each body reportingthat it felt like a part of itself was being ripped out. And so on. Noprofessional discussion of the case has been found, unfortunately, butit was widely reported in the press at the time, for example inTime, Apr. 6, 1981.

4.1.3 Hemi-neglect and Anosognosia

In hemi-neglect, one loses all sense of one side of one’s bodyor sometimes one half (divided vertically) of everything spatial inone’s experience. Whatever is going on in hemi-neglect, unifiedconsciousness seems to remain. It is just that its ‘range’has been bizarrely circumscribed. It encompasses an experience of onlyhalf the body or half of objects seen, not of the whole body or wholeobjects. Where we expect perception and proprioception of the wholebody and whole objects, these patients perceive and propriocept onlyone-half of the body and/or objects in general. So hemi-neglect isanother phenomenon in which there may be a major change in thephenomena over which unified consciousness without unifiedconsciousness itself being degraded.

Another phenomena in which unified consciousness seems to remain butwith a bizarrely circumscribed range is anosognosia. In thiscondition, a person who has suffered loss of function (often as aresult of a stroke) is unaware of the deficits (Gennaro 2015a). Thus,a person now blind will insist that she can see—and will stumbleabout in a room bumping into things. A person whose limbs are nowparalysed will insist that his limbs are moving—and will becomefurious when family and caregivers say that they are not. And so on.

4.1.4 Dissociative Identity Disorder

Another candidate phenomenon is what used to be called MultiplePersonality Disorder, now, more neutrally, Dissociative IdentityDisorder (DID). Everything about this phenomenon is controversial,including whether there is any real multiplicity ofconsciousness at all (Hacking 1995; Dennett 1998a). DID can taketwo forms. The more common form is often described as the units(persons, personalities, sides of a single personality, or whateverone decides to call them) ‘taking turns’, usually withpronounced changes in personality. When one is active, the other(s)usually is(are) not. Here the most prominent symptom is usuallystrange memory gaps (amnesias) in each ‘unit’ for periodswhen the body in question was clearly conscious and active. In theother, less common form, both ‘units’ are present at thesame time. Here, for example, the unit in control of speech willreport that another ‘person’ inside her is, say, givingher orders, these orders being experienced not from the standpoint ofgiving them but as coming from another person. This form of DID iscalled the co-conscious form in the literature but the term namessomething very different here from what James, Parfit and the like hadin mind when they used the term. Among other things, me and the‘little person inside me’ are not unified in oneconsciousness. In fact, sometimes the dissociation in both forms ofDID is behaviourally as complete as it is in brain bisection patientsin the lab.

4.1.5 Schizophrenia

In some particularly severe forms of schizophrenia, the victim seemsto lose the ability to have an integrated, interrelated experience ofhis or her world and self altogether. The person speaks in ‘wordsalads’ that never get anywhere, indeed sometimes never becomecomplete sentences. The person is unable to put together perceptions,beliefs and motives into even simple plans of action or act on suchplans if formed, even plans to obtain sustenance, tend to bodilyneeds, escape painful irritants, and so on. Here, it is plausible tosuggest that the unity of consciousness has shattered rather thansplit. The behaviour of these people seems to express what we mightcall mere experience-fragments, the contents of which are so narrowand unintegrated that the subject is unable to cope with daily lifeand interact with others in the ways that, for example, split brainsubjects can.

4.1.6 Dysexecutive Syndrome

In schizophrenia of the severe sort just described, the shattering ofconsciousness is part of a general breakdown or deformation of mentalfunctioning: affect, desire, belief, even memory all suffer massivedistortions. In another kind of case, the normal unity ofconsciousness seems to be just as absent but there does not seem to bethe same sort of general cognitive or affective disturbance. This istrue of what some researchers call dysexecutive syndrome (Dawson,1998: 215, for example). What indicates breakdown in the unity ofconsciousness is that these subjects are unable to consider two thingstogether, even things directly related to one another. For example,such people cannot figure out whether a piece of a puzzle fits into acertain place even when the piece and the puzzle are both clearlyvisible and the piece obviously fits. They cannot crack an egg into apan. And so on.

Trevarthen (1984) reports a similar syndrome in a few patients. In thecases he reports, commissurotomy patients are conscious of some objectseen in the right side of the visual field by the left hemisphere(controlled so that the information is received by only thathemisphere) until an intention is formed to reach for it with the lefthand, controlled by the right hemisphere. Somehow the intention toreach for it seems to obliterate consciousness of it in the hemispherethat controls speech, presumably the left hemisphere. However, if theobject is slid over to the left visual field, then thespeech-controlling hemisphere reports that it can see the objectagain—even though the object can now be seen only by the righthemisphere and the left still controls speech!

4.1.7 Simultagnosia

A disorder presenting similar symptoms is simultagnosia orBalint’s syndrome (Balint was an early 20th century Germanneurologist). In this disorder, patients see only one object locatedat one ‘place’ in the visual field at a time. Outside of afew degrees of arc in the visual field, these patients say they seenothing but an “undifferentiated mess” and seem to bereceiving no information about objects (Hardcastle 1997: 62).

What is common to dysexecutive disorder, Trevarthen’s cases, andsimultagnosia is that subjects seem not to be conscious of even twoobjects in a single conscious state. They cannot, for example, comparethe objects (in Trevarthen’s cases, the object of a perceptionwith the object of an intention). Unlike commissurotomy cases, it isnot the case that a conscious experience of the second item existswithin another unified consciousness. If there is any experience ofthe second item at all, it is not conscious. Rather than consciousnessbeing split into two discrete parcels, there is just one diminishedparcel. The rest of the conscious experiencing that is typical ofnormal consciousness has disappeared.

There are of course many different theories about what is going on inthe conditions we have just sketched, severe schizophrenia,dysexecutive syndrome, simultagnosia/Balint’s syndrome. Somehold that the deficits are not in unified consciousness at all; theyare in the capacity to process perceptual information. On this view,consciousness remains unified but patients can no longer can take inwhat is happening. Here we will explore only the idea that the problemis with unified consciousness. If it is even possible that this iswhere the problem is, we can learn interesting things about the unifyof consciousness.

Many of the disorders that we have considered are fairly directly aresult of changes to the brain. Cognitive neuroscience is the study ofthe relationship between the two. The best-known objects of such studyin connection with unified consciousness are brain bisectionoperations (commissurotomies). Other neuroscientific studies relevantto unified consciousness have examined blindsight (Weiskrantz 1986),blindsight with visual agnosia (van Gulick 1994), hallucinations andthought insertion (Stephens & Graham 2000), and archaic handsyndrome and delusions of control in schizophrenia (Mylopoulos 2015).

4.2 Do the Conditions have a Common Structure?

Can a single structure account for what is going on in all thesephenomena? The Kantian taxonomy of forms of unified consciousnessconsidered inSection 2.2can give us a way into this issue.

As we said earlier, one natural way to think of the conditions justsketched is to break them into two groups. In one group, howeverdrastic the change in unified consciousness, unified consciousnessremains in a largely complete form. Arguably, this group would includebrain bisection cases, hemi-neglect, anosognosia, and DID. In thesecond group, schizophrenia of the severe kind we sketched,dysexecutive disorder, and simultagnosia, it is more natural to thinkthat unified consciousness has been damaged or even destroyed.

Brain bisection cases first. On the standard way of conceptualizingthese cases, the key evidence for a duality of some kind is that thereappear to be situations in which whatever is conscious of some itemsbeing experienced in the body in question is not conscious of otheritems being experienced in that same body at the same time. We lookedat two examples of the phenomenon inSection 4.1,the word TAXABLE and the doing of arithmetic. With respect to theseexperienced items, there is a significant and systematicallyextendable situation in which to be conscious of some of these itemsis not to be conscious of others of them in a single unifiedconsciousness where we would expect such consciousness. If so, brainbisection patients fail to meet the conditions for unifiedconsciousness of contents. This seems to be what motivates thejudgment that these patients have two centres of consciousness.

Let us describe the case a bit more precisely. On the standardaccount, a brain bisection patient could be conscious that both TAXand ABLE are being seen. But nothing in the patient would be consciousthat both items are being seen on the basis of seeing them.For at least one of the two items, one of the two centres ofconsciousness could have only the same kind of behavioural and other‘outside’ evidence that any observer of the situationcould have.

Indeed, the apparent split runs still deeper. Between the twohemispheres there seems to be a split in unified consciousness ofexperiencing, too: Consciousness of doing some experiencinggoes with lack of consciousness of doing other experiencing that isgoing on in the same body. To use a useful metaphor first coined, sofar as we know, by Shoemaker, something is not conscious of acts ofexperiencing going on in its body ‘from the inside’, i.e.,on the basis of doing them, while something else is consciousof them on this basis. There seems to be a split in unifiedconsciousness of self, too: Consciousness of oneself assubject on the basis of doing acts of experiencing in that body goeswith lack of consciousness of oneself as subject on the basis of otheracts of experiencing being done in that body. If so, for manyconscious states in these patients, there two instances ofjoint consciousness (section 2.1.3),not the normal one.

To be sure, this assessment is not universally accepted. As we saw,Bayne and Chalmers (2003) and Bayne (2010: ch. 9) urge that whileaccess unity is split in these patients, phenomenal unity need not be.Why not? Because there may be no duality of unified consciousness atany given time. Rather, a single instance of unified consciousness maybe switching back and forth between the material in the twohemispheres. As Schechter (2012) has urged, evidence of a simultaneousduality of consciousness would be a major problem for thisapproach. We will not attempt to assess the relevant evidence here(see Brook 2015 for further discussion). Even if Bayne is right, thatshould not affect the prospects of a unified account of disorders ofunified consciousness—what is going on in brain bisection caseswould not be a disorder of unified consciousness!

Next, dissociative identity disorder (DID). (We will take uphemi-neglect and anosognosia shortly.) In cases of DID, a centralfeature is either some pattern of reciprocal amnesia or a strong sensethat another is inside (and yet still separate). This again seems tobe a situation in which being conscious of some experienced objects byhaving the experience goes with not being conscious of others in thesame body in the same way. The main difference is that the breachseems to be at a time in brain bisection cases, but can be eitheracross time or at a time in DID cases. If so, the breakdown in unitywill again consist in breakdowns of joint consciousness. The amnesiain diachronic DID has this character, clearly (and, except for theamnesia, is in line with Bayne’s (2010) switch model), but sodoes synchronic DID. The person and the ‘little personinside’ have no access from the inside to each other, so nocommon unified consciousness of experiencing, and the objects of theirexperience are not unified, so no common unified consciousness ofcontents.

Now the second group. For severe schizophrenia, dysexecutive syndrome,and simultagnosia/Balint’s syndrome—the cases whereconsciousness seems to be more shattered than split—thedistinction of Kant’s that we introduced inSection 2.2between two kinds of synthesis is useful. As we saw, Kantdistinguishes between the synthesis that gives us consciousness ofindividual objects (Tye’s object unity) and the synthesis thatgives us consciousness of a number of objects at the same time.

This distinction seems to shed some interesting light on the threephenomena. The evidence suggests that the first kind of synthesiscontinues to be available to dysexecutive and simultagnosia patients:they continue to be conscious of individual objects, events, etc. Thedamage seems to be with respect to the second kind, being conscious ofmultiple objects in a single act of consciousness. These people seemto achieve some measure of unified focal attention with respect toindividual objects but unified consciousness of multiple objects iseither restricted or missing.

With the severe forms of schizophrenia that we sketched, patients maylack even the ability to perform the first kind of synthesis. In adifferent jargon, these people may lack even the capacity for objectconstancy.

On this analysis, hemi-neglect and anosognosia are a bit differentfrom the other conditions. Here there is no apparent breach of jointconsciousness. Neither a split nor a breakdown in unifiedconsciousness is evident. Rather, in both conditions, there appears tobe a shrinking of the array of phenomena over which otherwise intactjoint consciousness can range. Half of one’s body and/or half ofall perceived objects are excluded in the first condition, the actualsituation with respect to sight, limbs, etc., in the second.

To sum up, it appears that there is some prospect of placing all theconditions we have considered within a single structure of (Kantian)distinctions, first between unified consciousness of contents andunified consciousness of experiencing, then between consciousexperience of individual objects and unified conscious experience ofmultiple objects. Thought insertion might pose a problem for thisscheme. Patients experience thought insertion when they believe thatsome of their thoughts, experiences, emotions, and so on are not theirown and have been ‘inserted’ into them from without.People suffering it are still aware of the ‘alien’thoughts from the point of view of having them. The deficit is insomething to do with thinking of these thoughts as unified with therest of conscious life. Normally, we think of ourselves as the subjectand agent of all our experiences. In thought insertion, the victimdoes not appear to himself to be either subject or agent of some ofthe experiences of which he is in fact the subject and agent(Mylopoulos 2015).

5. Unity Across Time

Unified consciousness at a given time (synchronic unity) has mainlybeen our topic so far. We now turn, more briefly, to unifiedconsciousness over time (diachronic unity). As was noted as long agoas Kant, unity across time is required even for such rudimentarymental operations as counting (1781: A103); indeed, unity across timeis crucial for virtually all cognition of any complexity. Now,unification in consciousness might not be the only way tounite earlier cognitive states (earlier thoughts, earlier experiences)with current ones but it is certainly a central way and the one bestknown to us.

5.1 Retention and Memory

In its synchronic form, we have suggested that a natural way to thinkof unified consciousness is in terms of joint consciousness.Diachronically, unified consciousness has an additional feature; itrequires retention over time, specifically, retention of earlierexperienced contents as one experienced them. What the retentioncrucial to diachronic unity consists in is a matter of some interest.It is tempting to assume that it is a kind of memory. However, asHusserl already told us, there is reason to be sceptical of thisapproach. There is a difference between experiencing asuccession from time 1 to time 2 and merely rememberingexperiencing what happened at time 1 while experiencing something attime 2. Dainton captures Husserl’s point by noting thedifference between “immediate and representedexperience—remembering or imagining hearing a tone is not thesame as directly experiencing the tone” (Dainton 2005: 155;Dainton cites Husserl 1928).

Kelly (2005) raises a similar question. Suppose that one is listeningto a melody. It has five notes and the final note is just beingplayed. If one simply recollected the earlier notes, one shouldexperience a chord, not five notes spread out and related to oneanother in time. Somehow, the earlier notes come‘date-stamped’ but still available to be integrated withthe current experience in a single, temporally-extended, unifiedexperience. Whatever this process is like, it is clearly vital to ourkind of unified consciousness. Without it, one could not hear anysequence as a sequence or so much as read a simple sentence. Thoughsome theorists call this across-time process unity of consciousness, amore distinctive name for it would be the continuity ofconsciousness.

This sort of continuity of consciousness can span very short durations(such as the ‘specious present’). Even a seemingly simple,current experience is in fact a continuous experience of more than oneinstant, and must be if one is to hear a sound or perceive(as opposed to remember) any temporally stretched phenomenon. How canone have a unified conscious experience (not just a memory) ofduration?

Here again the debate that we mentioned earlier over whether a unifiedconscious experience is one experience or an assembly of manyexperiences rears its head. Dainton (2005: chs. 5–7) takes acontinuous, unified experience to include co-conscious experiences asparts. Tye (2003: ch. 4) urges instead that a diachronically unifiedexperience has multiple contents but no experiences as parts.

5.2 Unity and Personal Identity

In the history of European philosophy at least since Locke, diachronicunified consciousness has been closely linked to personal identity inthe philosopher’s sense, i.e., continuing to be a single person,one and the same person, across time. (The point of the restriction tophilosophy is that clinical psychologists use the term quitedifferently, as the name for certain aspects of personality and‘self-conception’.) Whatever may be true of the kind ofdiachronic unity we just discussed, the kind of diachronic unityassociated with personal identity is clearly a kind of memory,specifically, a kind of autobiographical memory. At least since Locke,philosophers have argued that as far back as unified consciousnessvia the right kind of autobiographical memory extends, thereextends the person, one and the same person over all this time. Theright kind of autobiographical memory is memory of the having,feeling, or doing of earlier experiences, emotions, actions, and soon. As Locke has it, being the same person just is having the‘same consciousness’. We must be careful here.There is lots of autobiographical memory that is not memory from thepoint of view of experiencing. A person can remember that so-and-sohappened to her without remembering the event, the experience of it,or anything else ‘from the inside’, to useShoemaker’s useful metaphor again. Memory theorists’standard categories are not fine-grained enough for our purposes here.

Some important philosophers have urged that memory-carried diachronicunity is not sufficient for being one person over time. Kant, forexample, argued for a dissociation here, in his famous critique of thethird paralogism. In Kant’s view, continuity sufficient to“retain the thought of the previous subject and so hand it overto the subsequent subject” (1781: A363), continuity sufficienttherefore for diachronic unity of consciousness, is quite compatiblewith the ‘retained thoughts’ being passed from one subjectto another, compatible therefore with an utter absence of personalidentity. If so, diachronic unity is not sufficient for personalidentity (Brook 1994: ch. 8). (Note: Locke and Kant may be less farapart than this brief discussion would suggest. We are merely usingthem to illustrate the two positions, not discussing either of themfully.)

Phenomena relevant to identity in things other than persons can be amatter of degree. This is well illustrated by the famous ship ofTheseus. Suppose that over the years, a certain ship was rebuilt,board by board, until every bit of it has been replaced. Is the shipat the end of the process the ship that started the process? Nowsuppose that we take all those rotten, replaced boards and reassemblethem into a ship. Is this ship the original ship? It seemsthat there is no determinate answer to these questions. Say what youlike, and what you like may vary depending on whether you are aninsurance adjuster or a history buff. Many philosophers have insistedthat such indeterminacy can never be the case for persons. Identity inpersons is always completely unambiguous, not something that couldever be a matter of degree (Bishop Joseph Butler [1736] is awell-known example).

Brain bisection cases (described inSection 4.1)in which, some urge, unified consciousness splits in two may berelevant here (Parfit 1971, 1984). As Parfit argues, the possibilityof persons (or at any rate minds) splitting and re-fusing puts realpressure on intuitions about our specialness. Perhaps the continuityof persons can be just as tangled and just as much a matter of degreeas the continuity of any other middle-sized object.

Two final comments. Nagel (1971) argues that there can beindeterminacy in synchronic unity, too (seeSection 6). One can sympathizewith Parfit about diachronic unity and yet have reservations aboutNagel on synchronic unity. Likewise, one should distinguish thequestion of whether diachronic unity can be intransitive from thequestion discussed inSection 6of whether synchronic unity can be intransitive.

6. Two Philosophical Questions

Philosophers have made some fairly exotic claims about brain bisectioncases and related conditions. Here we will consider two of them.

6.1 No Whole Number of Centres of Consciousness

The first is a claim that in brain bisection patients, there is nowhole number of persons. So far, we have talked as though in brainbisection we always end up with some clear number of instances ofunified consciousness. Nagel (1971), one of the early philosophers towrite about these cases, rejects that view. For him, there is no wholenumber of ‘centres of consciousness’ in brain bisectionpatients: there is too much unity (for example in life outside thelaboratory and even in behaviour within) to say “two”, yettoo much separation in the specially contrived laboratory situationsto say “one”.

Not being happy with so counterintuitive a result, philosophers haveresponded. A response favoured by many is this. For anyprecise ‘one or two?’ question, there will be aprecise answer. Behavioural control system? One. Groups of experiencedobjects unified in consciousness? Two. And so on. If so, while theone’s and the two’s wouldn’t line up as tidily asthey do most of the time in people who have not had this operation, itis not obvious that the mixed answers support Nagel’s conclusionthat there may be no whole number of centres of consciousness in thesepatients.

6.2 Partial Unity is Possible

The ‘in some respects one, in some respects two’possibility at the centre of Nagel’s analysis is related to aquestion about transitivity. Is it possible, for a given instance ofexperienced objects p, q, and r, for there to beunified consciousness of p and q, unified consciousnessof q and r, but no unified consciousness of p andr? (The parallel in brain bisection cases? Call the‘centres of consciousness’ in the two cerebral lobesA and C, the older unilateral brain below them B.Is it possible for a mental state in A to be unified inconsciousness with one in B, one in B with one inC, and yet the state in A not to be unified with thestate in C?)

Hurley (1998) examines this question in detail, as does Bayne (2010:ch. 9.4). Hurley comes at the question of transitivity by consideringsome results reported by Sergent (1990). Since Sergent’s workhas not been replicable, let us look instead at how the research byTrevarthen that we mentioned inSection 4.1.6throws up the same issue. In this research, as we said, brainbisection patients under certain conditions are conscious of someobject seen by, say, the right hemisphere until the left hand, whichis controlled by the right hemisphere, reaches for it. Somehow the actof reaching for it seems to obliterate the consciousness of it. Verystrange—how can something pop into and disappear from unifiedconsciousness in this way? This question leads Hurley to the notion ofpartial unity. Could two centres of consciousness, Aand C, though not unified in consciousness with one another,nonetheless both be unified with some third thing, in this case thevolitional system B (the system of intentions, desires, etc.)?If so, ‘being unified with’ is not a transitiverelationship—A could be unified with B, andC could be unified with B, without A beingunified with C. This idea is puzzling enough. Even morepuzzling is how activation of the system B, with which bothA and C are unified, could result in the loss ofconsciousness in A and/or C of an object aimed at byB.

Hurley never pronounces on the possibility of partial unity. Instead,she argues that none of the cases suspected of displaying it reallydo. She accepts that intention can obliterate consciousness—butthen distinguishes time periods (1998: 216). In Trevarthen’scases, for example, the situation with respect to unity is clear atany given moment—one either is or is not conscious of theobject. The picture over time does not conform to our usualexpectations for diachronic singularity or transitivity ofunity—but that is simply an artefact of the cases, not aproblem.

Hurley considers another class of cases, what she calls Marcel’s(1994) cases. Here subjects are asked to report the appearance of someitem in consciousness in three ways at the same time—say, byblinking, pushing a button, and saying, ‘I see it’.Remarkably, in different trials each of these three acts are donewithout doing the other two. And the question is, What does this implyfor unified consciousness? In a case in which the subject pushes thebutton but neither blinks nor says anything, for example, is thehand-controller aware of the object while the blink-controller and thespeech-controller are not? How could the conscious system becomefragmented in such a way?

Hurley’s suggestion? They can’t. What induces theappearance of incoherence about unity is the short time scale. Supposethat it takes some time to achieve unified consciousness, perhapsbecause some complex feedback processes are involved. If so, thenMarcel’s cases have not got to a stable situation with respectto unity. The subjects were not given enough time (1998: 216).

Is partial unity possible? To date, this remains an unansweredquestion, though Bayne (2010: 209) leans towards a negativeanswer.

Hurley discusses more aspects of the unity of consciousness thanpartial unity. She argues, for example, that there is a normativedimension to unified consciousness—conscious states have tocohere semantically for unified consciousness to result (we willreturn to this issue inSection 7.3).She discusses most of the kinds of breakdown phenomena that weconsidered earlier, exploring the implications of a wide range of‘experiments of nature’ and laboratory experiments for thepresence or absence of unified consciousness. In particular, sheconsiders acallosal people (people born without a corpus callosum).Even though the corpus callosum, when present, is the chief channel ofcommunication between the hemispheres, acallosal people show all thebehavioural signs of having fully unified consciousness. If so, thenthe neurological and behavioural basis of unified consciousness wouldbe very different in different people. (We will return to this lasttopic inSection 8.)

7. Theories of Unity

In the literature, there is quite a range of theoretical claims aboutunified consciousness. We have looked at some of them. The first groupconcerned diverging models: the subsumption, co-consciousness, singlephenomenal content, and joint consciousness models and relatedtaxonomies. Then we looked at the unity thesis, claims about limits toand disorders of unified consciousness, claims about unity over time,and claims that there need not be a whole number of centres ofconsciousness and that partial unity is possible. InSection 2,we said that we’d return to the issue of whether unifiedexperiences has or does not have experiential parts (EP vs. NEP), theissue that underlays subsumption. EP and NEP are the topics ofSections7.1and7.2.Near the end ofSection 4,we said that we would return to the claim that unified consciousnessrequires links among conscious contents. We will take up this issuesinSection 7.3.Finally for this Section, we will examine a claim that we have notdiscussed so far, what is usually called the co-ownership thesis(Section 7.4)

7.1 The Experiential Parts Theory

How is unified conscious experience structured? As we mentioned inSection 2.1,two incompatible models have some currency at the moment. On theexperiential parts view (EP), unified conscious experience includessimpler experiences as parts or something like parts; unifiedconsciousness has a mereological aspect. On this view, when I have aunified experience of a pain and a noise, this unified experienceincludes an experience of just the pain, and an experience of just thenoise. These simpler experiences are the relata of unifiedconsciousness; they are joined as parts of the unified experience ofthe pain and noise together. Experiences a and b areunited in a third experience, c, which is their jointoccurrence. On the no experiential parts (NEP) account, the consciousmental act through which diverse contents are presented does not haveother conscious states, experiences, as parts. On the first view, whenI have unified consciousness of experiencing, I am conscious of manyexperiences. On the second view, I am conscious of just oneexperience.

To clarify the two, let us use the notation ‘\(E(o_1)\)’for an experience that is the conscious experience of just theintentional object \(o_{1}\). A conscious experience of just \(o_{2}\)is \(E(o_{2})\). What is the nature of an experience that takes thebigger content in which \(o_{1}\) and \(o_{2}\) are presentedtogether? On NEP, it has the structure of \(E(o_{1}, o_{2})\), wherethis introduces a single experience that has both contents as itsobject. To be conscious of \(o_{1}\) by means of this experience is tobe conscious of it with \(o_{2}\). (See the concept of jointconsciousness introduced inSection 2.1.3.)According to NEP, this is what the subject’s conscious unity atthe time amounts to (if we oversimplify by supposing her to beconscious of nothing but \(o_{1}\) and \(o_{2}\)). No‘smaller’ or simpler conscious states figure as parts.This experience might be realized in a brain state that has parts, butthese parts are not further conscious states. By contrast, in EP\(E(o_{1})\) and \(E(o_{2})\) persist as parts of an encompassingexperience by means of which one is conscious of \(o_{1}\) and\(o_{2}\) together and unified consciousness would have the structure\(E(E(o_{1}) E(o_{2}))\).

Proponents of EP include Lockwood, Dainton, Shoemaker, and Bayne andChalmers, and Bayne by himself (in his 2010 book). As we saw inSection 2.2,theorists as otherwise different as Dainton, Lockwood, and Shoemakeruse the term ‘co-consciousness’ as the name for therelationship that ties the experiential parts together. Not only aremost versions of EP built on some notion of co-consciousness; mostnotions of co-consciousness assume EP and are incompatible with NEP.

EP faces a difficulty. James describes the problem in his example ofthe twelve-word sentence. Suppose each word in the sentence is knownby just one of twelve people. It is hard to see, James says, how thesetwelve thoughts could be combined to yield a unified consciousness ofthe sentence. As he says,

Take a sentence of a dozen words, take twelve men, and to each oneword. Then stand the men in a row or jam them in a bunch, and let eachthink of his word as intently as he will; nowhere will there be aconsciousness of the whole sentence. (James 1890: 160)

What EP needs is a way of combining experiences that does not simplyconjoin them into an experiential aggregate, for a mere combination ofexperiences is not the experience of a combination. EP needs, then, away of putting together experiences that also puts together theircontents. Without any specification of how this combining of contentsis to be achieved, we are left with a mere aggregate of experiences,each member of which is oblivious to the contents of the other statesin the aggregate. As James puts it, “Idea of a + idea ofb is not identical with idea of (\(a + b\))” (1890:161).

Proponents of EP may reply that they never intended to give anaccount of conscious unity. Instead, EP should be taken toprovide a description of the structure of unified consciousstates. In other words, it is a characterization, not an explanation,of such states and how they are individuated. It may be held that theonly good way to individuate conscious states is on the basis ofcontents. Hence, since one is aware of many contents via a consciousstate, that state must itself consist of several simpler consciousstates that have (somehow) come to be unified. At the very least,then, those who advocate NEP owe us an alternative basis forindividuating conscious states. We will return to this issue in thenext section.

As we saw, the notion of a unified experience subsuming simplerexperiences is central to Bayne’s and Chalmers’ (2003; seeBayne 2010) account of the unity of consciousness. This may appear tobe a version of EP and Tye treats it so (without using that label;2003: 21). However, Bayne and Chalmers hedge their bet. While they dospeak of the encompassing conscious state as involving “at leasta conjunction of each of many more specific conscious states”(2003: 27), and of a “complex phenomenal state and a simplerstate that is intuitively one of its ‘components’”(2003: 40), they also caution that thinking here in terms of “amereological part/whole relation among phenomenal states” shouldbe regarded only as an “aid to intuition rather than as aserious ontological proposal” (2003: 40). So how their viewstacks up with respect to EP is not entirely clear.

7.2 The No Experiential Parts Theory

Searle and Tye are leading current advocates of NEP. Searle ([2000]2002: 56) ventures that “maybe it is wrong to think ofconsciousness as made up of parts at all”. For one has a“single, unified, conscious field containing visual, auditory,and other aspects” and “there is no such thing as aseparate visual consciousness” ([2000] 2002: 55). Do“visual experiences stand to the whole field of consciousness inthe part-whole relation?” ([2000] 2002: 54). No, says Searle(though he may do some backsliding later in the article).

Tye offers a similar view, which he dubs the ‘one-experienceview’ (Tye 2003: ch. 1). Considering the polymodal nature of ourexperience, he says, “There are not five different …experiences somehow combined together to produce a new unifiedexperience”. Instead, “there is just one experiencehere” (2003: 27).

Part of what is at stake in this dispute is how to individuate (how tocount) experiences. EP theorists think that experiences go one to anobject: if you experience two things, you have two experiences. Oreven finer: if you experience one thing in two ways, you have had twoexperiences. NEP theorists hold that some experiences can beindividuated differently; a unified act of conscious experiencing is asingle experience, experientially non-composite, no matter how manyobjects it has.

So what are the arguments for the two views like? Theorists who acceptEP usually just assert or even assume it (we saw some examples of thisinSection 7.1).The idea just seems intuitively plausible to its adherents and theytend not to argue for it. NEP is not intuitively obvious—evenpeople such as Searle who advocate it can find themselves sliding intoEP—and its adherents do argue for it.

James was the first champion of NEP. He endorsed it in the course ofrepudiating the ‘mind-stuff theory’, according to which“our mental states are composite in structure, made up ofsmaller states conjoined” (1890: 145). Against this James saysthat, while our experience is complex, this complexity is not a matterof there being several experiences (or‘feelings’) present in an encompassing experience. This isbecause “we cannot mix feelings as such, though we may mix theobjects we feel, and from their mixture get newfeelings” (1890: 157). If one’s experience appears tobecome more complex, that is a matter of a singleexperience’s content being more complex, and is not theaddition of more experiences (of the diverse contents).Indeed, he says, “We cannot even … have two feelings inmind at once” (1890: 157).

Here is how this is supposed to work. If we say that experiencesa and b are fused to form experience c, we shouldtreat ‘fused’ as referring to a process in which aand b are superseded by c, not included in it. Aand b have been replaced by c, in which their contentsare connected, and they (a and b) no longer exist. AsJames put it, contrasting the unified consciousness of the wholealphabet with the several states involved in consciousness of eachletter taken singly,

It is safer … to treat the consciousness of the alphabet as atwenty-seventh fact, the substitute and not the sum of thetwenty-six simpler consciousnesses. (1909: 189)

This view clearly avoids the problem of how to combine experiencesthat faces EP.

Since James had a concept of co-consciousness and we have linkedco-consciousness closely to EP, we should say a word about hisconcept. It is not the same as the concept that we find in Parfit,Lockwood, Hurley, Shoemaker et al. For James, co-consciousness relatesonly to a multiplicity of items of which one is conscious. In unifiedconscious experience of them, there is no multiplicity of consciousstates to enter into a ‘co’-relationship. Indeed, thecontents are made co-conscious by being presented together in asingle, noncomposite experience.

We will close this discussion with two notes about the relationship ofthe dispute between EP and NEP to the transparency thesis that wediscussed earlier. This is the thesis that we are not directlyconscious of our own experiences. (‘Transparent’ heremeans that while I am conscious via conscious states, I amnot conscious of them. I ‘see through’ them, asit were; hence ‘transparency’.) First, all claims for bothEP and NEP that we have considered would seem to go through even ifthe transparency thesis were true. So transparency would not underminethis debate. Secondly, even Tye (2003), who accepts both NEP andtransparency, also accepts that NEP does not require orentail transparency. Rather, he seems to think that thetransparency thesis is true and, since it is true, this constrainswhat could be unified in phenomenal unit. Accepting the constraint, hehas to say that, “Phenomenal unity is a relation betweenqualities represented in experience, not between qualitiesof experiences” (Tye 2003: 36), however unintuitivethis claim may be. Certainly there are approaches that can both acceptNEP and reject transparency. That is true, for example, of those whohold that conscious states are self-representing states, states ofwhich one becomes conscious just by having them. These theorists willhold that one can be directly conscious of one’s ownconscious states, yet they could still hold that states of unifiedconsciousness have an NEP structure.

7.3 The Internal Links Theory

Now we turn to the idea that unified consciousness of contents andexperiencing requires some kind of phenomenally evident relation amongthe contents of the unified conscious state (in addition to thecontents being aspects of a single unified act of consciousness) andsome attempts to model this relationship. We might call this therelational model of unified consciousness. It is a descendantof Kant’s claim examined inSection 1that unified consciousness requires conceptual interconnectedness inthe objects of consciousness; as he put it,

all appearances stand in a thoroughgoing connection according tonecessary laws, and hence stand in a transcendental affinityof which the empirical affinity is the mere consequence. (1781/1787:A113–114)

Kant’s argument for this claim seems to have been that synthesisof represented objects to produce a single complex object is anecessary condition of consciousness of self as single common subject.Without represented objects being tied together in a single complexobject, one might be aware of the subject of an individualrepresentation but one could not be aware of the subject of one suchrepresentation as the subject of other such representations. Rather, Ishould have “as many-coloured and diverse a self as I haverepresentations of which I am conscious ..”. (B134)—as arein fact had by me, for I would not, of course, be aware that it wasme.

One recent expression of the idea is Hurley’s (1998) claim thatconscious states must satisfy a normative requirement if unifiedconsciousness is to result. Specifically, they must ‘coheresemantically’. (It is not entirely clear that satisfying thisnormative requirement requires that the contents of consciousness belinked in the way that Kant urged.) Even more recently, Revonsuo(2003) has urged that phenomenal contents must be situated in the same‘phenomenal space’ in order to be unified, adding,“I am inclined to treat phenomeno-spatiality as the basicunifying feature of human consciousness”.

For theorists of this persuasion, these phenomenally evident spatial,causal, etc., relations among contents explain why my perceptualstates typically present one coherent world in which, for instance, awall in front of me and starting at one end of my visual field willcontinue across that field. Are such connections among the contents ofan experience necessary for their being presented together inexperience? Some recent theorists have argued that they are not (Bayne2004; Brook 2005). One can have unified consciousness of a siren thatthat one is hearing, an average grade that one is calculating, and afictitious landscape that one is visualizing. In what possible waycould items as diverse as these have to be connected to one another?They are not even all in space or contiguous to one another and theyare certainly not causally interconnected, yet I can experience allthree in a single act of unified consciousness.

Doesn’t there have to be at least logical coherence among thecontents? As we just saw, Hurley (1998) makes this claim, as doesBaars (1988). Hurley, for example, argues that we cannot believemutually inconsistent things when we are conscious of both in a singleunified experience. Could the disconnect among unified consciousstates extend to them actually being inconsistent with one another?Contrary to Hurley and Baars, the evidence suggests that it could.Suppose that one sees a stick immersed in water as being bent butfeels it to be straight or knows that this is an illusion. Here,one’s conscious perception that it is bent conflicts withone’s conscious belief that it is not bent, yet these states areunified in one consciousness (Bayne 2000). Tye (2003: 38) does notconsider this illusion to be an example of incoherence. He holds thathere touch corrects vision, making the stick appear to be straight,and so belief renders the appearance mere appearance. That claim iscontroversial. Bayne (2004: 227) discusses another example, invertingspectacles, more plausibly. These glasses render one’s visualcontents inconsistent with one’s tactile contents, yetconsciousness of the visual contents remains unified withconsciousness of the tactile contents.

In fact, it would seem that there can be incompatibilities even withina perceptual modality. Thus, Tye (2003: 38–39) notes that thereare pictures that depict impossible situations. He also discusses thewaterfall effect, in which, after staring at a waterfall for sometime, if one looks at the adjacent rock face, a portion of the rocksurface will appear to be moving—and not moving (relative to thearea around it; not everyone accepts that both elements aresimultaneously present in the waterfall illusion; see Crane 1988).These examples suggest that we can have unified consciousness ofpretty much any collection of items whatsoever, no matter how they arerelated to one another. Indeed, it is not easy to specify any relationamong unified contents or acts of experiencing (beyond their beingunified) that is required for them to be unified. If so,Hurley’s (1998) suggestion that meeting a normative requirementis necessary for unified consciousness, specifically, a requirementthat the experienced properties of things in unified consciousnesscohere semantically, is in trouble.

7.4 The Co-Ownership Theory

Finally, there is a model that holds that unity of conscious statesconsists in their ownership by a single subject, Bayne’s (2004)co-ownership. Is co-ownership meant to be necessary, or sufficient, orboth? The idea that unified conscious experience must be had by asingle subject could be trivially true, as it would be if the subjectat a time is just defined as a set of unified contents. For the thesisto become interesting, advocates of it would have to offer a richerconception of the subject than that. On such a conception, the claimthat experiences are had by the same subject would involvetheir attribution to the same extra-phenomenal substrate or bearer ofexperiences, one that can be individuated independently of what is tobe found in experience, and thus independently of the notion of aunified field of conscious contents. Stated thus, the thesis would notbe trivial and may well state a necessary condition of unifiedconsciousness.

However, if co-ownership is necessary, is it also sufficient? It seemsthat is is possible for items that are not unified with oneanother to be simultaneously presented to one and the same subject.This seems to happen, for example, in split brain cases, or fictionalvariations thereof—e.g., Parfit’s example of a singlesubject who is able simultaneously to try out two alternativeapproaches to solving a math problem (1984: 246–248). This stateof affairs would seem to be a case of parallel but nonunifiedsets of conscious states had by the same subject, in somegood sense of the term ‘same subject’. The same would seemto hold of the contents of a subject who switches back and forthbetween lobes who lacked memory of earlier contents. If so,co-ownership would appear not be sufficient for unity. Afortiori, co-ownership would appear not to capture what isdistinctive about unified consciousness. Put differently, ifthere is a requirement here of any kind, more than co-ownership seemsto be needed. The subject must have a certain kind of relationship tothe material. As Kant put it, the material must be something to thesubject (A116)—and this requires a subject to whom things can besomething. Mere ownership by itself would appear to fall short (Brook1994: 135–9, discusses these issues in connection withKant).

Thought insertion may pose another problem for the co-ownershipthesis. In thought insertion, the alien states are unquestionablyco-owned with the avowed ones but there seems to be less than fullunity of consciousness.

InSection 5.2,we examined Kant’s claim that instances of diachronic unifiedconsciousness are able to extend beyond one person (by any criterionfor being one person other than unified consciousness). If so,diachronic unity is not sufficient for singleness of person. On thestandard account, brain bisection cases suggest roughly the reverseclaim about synchronic unity; where there seem to be two instances ofunity in one body, by many criteria there is just one person(remember, there is no question about singleness of person outside thelaboratory). If so, singleness of person is not sufficient forsynchronic unity.

8. Neural Architecture of Unified Consciousness

We will conclude this article with a brief look at some philosophicalspeculations about what the neural architecture of unifiedconsciousness might or must be like. One of the hottest issues incurrent consciousness research is the issue of how brains achieveconsciousness and what parts of the brain are most involved in doingdo, what the ‘neural correlates of consciousness’ (NCC)are. Any real insights into the NCCs of consciousness in general arealso likely to contain insights into the NCCs of unifiedconsciousness. This literature is now so vast that it would take awhole additional article to discuss the topic. (Koch 2004 is anexcellent review of the empirical neuroscience and Chalmers 2000 isthe most extensive exploration of the conceptual issues to date.)Because of space limitations, here we will restrict ourselves to threeof the most influential philosophical approaches to what the neuralarchitecture of consciousness might be like, those of Paul andPatricia Churchland (see for example Paul Churchland 1995: 214),Daniel Dennett (1991), and Susan Hurley (1998).

The Churchlands’ view flows from a radical picture of neuralarchitecture in general. They urge that the architecture of theprocesses underlying cognition and consciousness consists not oftransformations of symbolically encoded representations, as mostphilosophers have believed, but of something like vectortransformations in phase spaces. Thus on their view, neural correlatesof unified consciousness are nothing remotely symbol- or sentence-like.

Dennett articulates an even more radical view, on both unity and thearchitecture of it. For him, unified consciousness of‘self’ is simply a short-lasting ‘virtualcaptain’ coming to be as a result of a small group ofinformation-parcels gaining temporary dominance in a struggle withother such groups for control of such cognitive activities asself-monitoring and self-reporting. We take these transient phenomenato be more than they are because each of them is the ‘me’of the moment and they are tied to earlier transient selves by thespecial form of autobiographical memory identified earlier. If thetemporary coalition of conscious states that is winning at the momentis what I am, is the self, each temporal chunk of ‘self’is likely to be found in different parts of the brain from other suchchunks and there will be many NCCs of unified consciousness in manydifferent places.

If Dennett is right, there would be reason for scepticism about whatHurley calls the isomorphism hypothesis. The isomorphismhypothesis is the idea that a given kind of change in consciousnesswill always reflect, even be the result of, a given kind of change inthe brain. Hurley is sceptical about it, too. One way in whichskepticism about this hypothesis arises in her work is viaconsideration of acallosals (people born without a corpus callosum).Even though the corpus callosum, when present, is the chief channel ofcommunication between the hemispheres, acallosal people show all thebehavioral signs of having fully unified consciousness. If so, it hasto be achieved by mechanisms such as cuing activity that are utterlydifferent from communication though a corpus callosum. And the samecan be true the opposite way. Different changes in consciousness cango with the same changes to structure and function in the brain. Andif both these claims are correct, then the neurological/behavioralbasis of unified consciousness would be very different in differentpeople. The isomorphism hypothesis would be false, attractive thoughit has been to many people.

Dennett’s and the Churchlands’ views fit naturally withina dynamic systems view of the neural implementation of cognition andconsciousness, the view that unified consciousness is a result ofcertain self-organizing activities in the brain and interactionsbetween brain and world. Dennett thinks that, given the nature of thebrain, which is nothing more than neurons sending and receivingsignals to and from other neurons, consciousness could not take anyform other than something like a pandemonium of competing bits ofcontent. The Churchlands don’t agree with Dennett about this.They see consciousness as a state of the brain, the‘wetware’, not a result of information processing, of‘software’. Both sides in this debate agree that it isunlikely that the processes that subserve unified consciousness aresentence-like or language-like.

9. Conclusions

A great deal of work has been done on the unity of consciousness inthe past few decades. Our introduction to it has been grouped aroundthe following themes:

• In some form, the unity of consciousness is a pervasive,cognitively important feature of our kind of mind.
• Even phenomenal unity of consciousness at a time comes in a numberof forms and consciousness is also unified across time.
• The ways in which unified consciousness can break down raiseinteresting questions about the phenomenon and throw important lighton its structure.
• The topic connects to a number of important issues concerning therelationship of consciousness to cognition, including whether unifiedconsciousness across time plays a role in personal identity (thephilosophers’ concept of personal identity).
• All the leading theories of the unity of consciousness faceproblems.
• The state of theorizing on the topic suggests that there is stillmuch room for further work.