Posted by: Xhyra Graf | 20 January 2007

Unity of Consciousness

On the other side, there are theorists who maintain not only that conscious states in a subject are unified but that they must be unified. Bayne and Chalmers (2003, p. 24) call this the unity thesis: necessarily, any set of conscious states of a subject at a time is unified. As we saw, Brentano probably held this view. Hill (1991) does, too. Bayne and Chalmers express sympathy for the thesis on the grounds that, “It is difficult or impossible to imagine a subject having two phenomenal states simultaneously, without there being a conjoint phenomenology for both states.” (p. 37) Merely having phenomenal states might seem too little but Bayne and Chalmers are talking about phenomenal states where, for them, to have the state is for the state to be like something. If we recast to make this element explicit, we get a claim of some real intuitive appeal: If A is like something to S and B is like something to S, it must be the case that the combination, A and B, is like something to S. Interestingly, Kant seems to have believed something similar: “[Experiences] can represent something to me only insofar as they belong with all others to one consciousness” (A116). A and B having this feature is exactly what unity consists in, according to Bayne and Chalmers and put this way, the unity thesis has some real appeal.

Are there presumptive counter-examples to the unity thesis? Spelled out as we have spelled it out, we do not know of any. Bayne and Chalmers consider brain bisection cases to be putative counter-examples because, on some concepts of the subject of experience, we can think of there still being one subject in these cases even though not all the conscious states are unified. There are at least three ways to respond. The simplest is just to deny that there is one subject, at least for the period of the split. A second would be to note that, however one counts subjects during the period of the split, there is evidence that many conscious experiences in that body are not like anything to some subject. If so, the apparent lack of conjoint consciousness of them will not be a problem. A third (advocated by Bayne and Chalmers, pp. 38-9) would be to urge that while there is clearly a breach in the unity of access consciousness (access to information for purposes of belief formation, behavioural control, and so on) during the period of the split, unified phenomenal unity may still extend across all the conscious experience. Unity at a time and across time

Unified consciousness at a given time (synchronic unity) has mainly been our topic so far. We now turn, more briefly, to unified consciousness over time (diachronic unity). As was noted as long ago as Kant, unity across time is required even for such rudimentary mental operations as counting (1781, A103); unity across time is crucial for all cognition of any complexity. Now, unification in consciousness might not be the only way to unite earlier cognitive states (earlier thoughts, earlier experiences) with current ones but it is certainly a central way and the one best known to us.

5.1 Retention and memory

Diachronically, unified consciousness has an additional feature; it requires retention over time, specifically, retention of earlier experienced contents as one experienced them. What the retention crucial to diachronic unity consists in is a matter of some interest. It is tempting to assume that it is a kind of memory. However, as Husserl already told us, there is reason to be sceptical of this approach. There is a difference between experiencing a succession from time 1 to time 2 and merely remembering experiencing what happened at time 1 while experiencing something at time 2. Dainton captures Husserl’s point by noting the difference between “immediate and represented experience — remembering or imagining hearing a tone is not the same as directly experiencing the tone” (Dainton 2000, p. 155; Dainton cites Husserl 1928).

This sort of continuity of consciousness can have to do with very small durations (such as the ‘specious present’). The central insight is that even a seemingly simple, current experience is in fact a continuous experience of more than one instant, and must be if one is to hear a sound or perceive (as opposed to remember) any temporally stretched phenomenon at all. How can one have a unified conscious experience (not just a memory) of duration?

5.2 Unity and Personal Identity

Whatever may be true of the kind of diachronic unity we just discussed, the kind of diachronic unity associated 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 consciousness via the right kind of autobiographical memory extends, there extends the person, one and the same person over all this time. The right kind of autobiographical memory is memory of the having, feeling, or doing of earlier experiences, emotions, actions, and so on. 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 the point of view of experiencing. One can remember that so-and-so happened to her without remembering the event, the experience of it, or anything else ‘from the inside’, to use Shoemaker’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 diachronic unity is not sufficient for being one person over time. Kant, for example, argued for a dissociation here, in his famous critique of the third paralogism. In Kant’s view, continuity sufficient to “retain the thought of the previous subject and so hand it over to the subsequent subject” (1781, A363), continuity sufficient therefore for diachronic unity of consciousness, is quite compatible with the ‘retained thoughts’ being passed from one subject to another, compatible therefore with an utter absence of personal identity. If so, diachronic unity is not sufficient for personal identity (Brook 1994, Ch. 8 ). (Note: Locke and Kant may be less far apart than this brief discussion would suggest. We are merely using them to illustrate the two positions, not discussing either of them fully.)

Two final comments. As we saw, Nagel (1971) argues that there can be indeterminacy in synchronic unity, too. One can sympathize with Parfit about diachronic unity and yet have reservations about Nagel on synchronic unity. Likewise, one should distinguish the question of whether diachronic unity can be intransitive from the question discussed in Section 4.3 of whether synchronic unity can be intransitive.


Brook, Andrew, Raymont, Paul, “The Unity of Consciousness”, The Stanford Encyclopedia of Philosophy (Fall 2006 Edition), Edward N. Zalta (ed.), URL =



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