Chris Mortensen, Gerard O'Brien & Belinda Paterson (1993) Distinctions: Subpersonal and Subconscious. Psycoloquy: 4(62) Split Brain (5)

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Psycoloquy 4(62): Distinctions: Subpersonal and Subconscious

Commentary on Puccetti on Split-Brain

Chris Mortensen, Gerard O'Brien & Belinda Paterson
Department of Philosophy
University of Adelaide
North Terrace, South Australia
5001 Australia


Puccetti argues that Dennett's views on split brains are defective. First, we criticise Puccetti's argument. Then we distinguish persons, minds, consciousnesses, selves and personalities. Then we introduce the concepts of part-persons and part-consciousnesses, and apply them to clarifying the situation. Finally, we criticise Dennett for some contribution to the confusion.


cartesianism, cell death, cerebral dominance, consciousness, hemispherectomy, lateralization, mental duality, mental unity, multiple drafts, split brain.
1. Towards the end of "Consciousness Explained," Dan Dennett applies his "multiple drafts" theory of consciousness to the issue of selfhood (1991, ch. 13). The conclusion he reaches is that selves are neither independently existing "soul pearls" (Cartesian dualism) nor material entities occupying specific points of space and time (Cartesian materialism). Rather, selves are biographical abstractions: centres of narrative gravity made up by the myriad of specialised cognitive agents that are housed in human brains (1991, pp. 426-7). In "Dennett on the Split-Brain" (1993), Roland Puccetti is concerned to respond to Dennett's claim that commissurotomised subjects only ever exhibit dual selves for brief periods of time during carefully devised experimental procedures: outside these situations, according to Dennett, split-brain subjects possess as robustly unitary selves as do normal humans. It is not surprising that Puccetti feels uncomfortable with this account, since, according to his own well-known position, not only are there two selves permanently housed in the split-brain subject, but "even in the normal, cerebrally intact human being there must be two persons [i.e., selves], though before the era of commissurotomy experiments we had no way of knowing this" (1973, p. 351). In this commentary, we begin by making some critical remarks about Puccetti, then offer some distinctions which we believe clarify the issues, and conclude with some brief criticism of Dennett.

2. Although Dennett's conception of selfhood is certainly controversial, Puccetti does not succeed in showing that it is untenable. This, we suggest, is largely because Puccetti has misunderstood Dennett's position. When Dennett writes of the absence of a permanent right-hemisphere self in the split-brain subject, Puccetti misreads this as the absence of a permanent right-hemisphere self simpliciter. As a consequence, Puccetti takes Dennett to be claiming that the right hemisphere is incapable of supporting a fully fledged self, a claim Dennett is not committed to. That Puccetti has misunderstood Dennett can be argued as follows. Consider Puccetti's discussion of the contrast between left hemispherectomy (left hemisphere removed) and neocortical death (neocortex removed). According to Puccetti, Dennett is committed to the view that "the left hemispherectomy patient is no better off than the neocortical death patient... there is `no one home' in either case" (para. 20), which Puccetti finds grossly counterintuitive. But Puccetti's remarks here are already in conflict with his own earlier discussion of Dennett's account of commissurotomy; for, as Puccetti notes, Dennett does allow a transitory right-hemisphere-based centre of narrative gravity to be possessed by split-brain patients in experimental situations. So Puccetti must accept that on Dennett's conception of selfhood, the right hemisphere is capable of supporting an independent self, albeit only briefly. If Puccetti accepts that Dennett's position allows for transitory right-hemisphere-based selves in the case of commissurotomy, then he should also accept that Dennett's position allows for fully fledged selves in the case of hemispherectomy. This is because the conditions which allow the development of a momentary right hemisphere self in the former case likewise allow the development of a permanent self in the latter case. What conditions are these? Merely the conditions whereby the right hemisphere, as opposed to the whole brain, is allowed to compose an independent biography, and thus become a centre of narrative gravity in its own right. Nor is there any reason to assimilate the morality of killing such a subject to the killing of a decorticated subject; and, contrary to Puccetti, Dennett is not committed to it.

3. At this point, it is worth making a distinction between persons, selves, minds and consciousnesses, if only because it is conceivable that the discussion will take different turns for each of them. Given this distinction, the first question to be asked is: how many persons are there post-commissurotomy? Now it has always seemed to us that there is available a simple answer for the case of persons: no persons but two part-persons. (We will qualify this suggestion later.) Any reasonable theory of what it takes to be a person calls for a multiplicity of functions or faculties to contribute to making up a person. Hence, if one entity almost entirely fails to possess the capacity to think linguistically, this is such a drastic deficiency in a central human faculty that one should hesitate to hold that we have here a person at all. The point is similar if what is lacking is the entire faculty of spatial reasoning. Notice that we say "part-person" rather than "person-part." This is to deal with the point, which obviously concerns Puccetti, that these are not any old parts of persons that we are dealing with. A part-person is a part of a person, yet so is a big toe; but a part-person is a lot more like a whole person than a big toe is: it is person-like. Perhaps we can say that part-persons are identified by characteristics featuring centrally in folk and cognitive psychology, whereas not all parts of persons are (e.g., big toes). But this does not entail that part-persons are (whole) persons. The same points can be made, we believe, when "persons" is replaced by "minds" or "selves." Minds are bundle-like things, consisting of many capacities and functions, so one can naturally allow for part-minds coexisting in the same body. The concept of a self is perhaps harder to get a grip on; on the other hand, there is no obvious reason one should not speak of part-selves likewise.

4. This suggestion about persons receives some indirect support from another cognate concept: personalities. We don't have much difficulty in attributing multiple personalities to a single body (though obviously under unusual circumstances). The situation is not exactly parallel to persons, because attributing multiple personalities does not immediately issue in the conclusion of multiple persons. Personalities are parts of persons also, particularly those parts having to do with character and emotional traits; but one might readily hold that one person can exhibit multiple personalities. This also indicates that our opening question might have different answers for different concepts.

5. So then: How many consciousnesses? This question looks harder, hence different, for two reasons: (1) because the phenomena surrounding splitting into part-persons might, indeed do, involve unconscious processes, so that one part-person might be conscious and the other not; and (2) because a part-person might not be a part-consciousness. After all, what could a part-consciousness be? Consciousness looks to be a lot less divisible than personhood. So, if one thought that there were two consciousness-like entities but that there was no such a thing as a part-consciousness, then one would presumably conclude that there are two consciousnesses. We suggest that this may be one of the main motivations behind Puccetti's views. Even so, part-consciousnesses are not such an absurd notion. There is an obvious sense in which even something as narrow as visual consciousness has parts, namely, the parts of the visual field. There is another obvious sense in which one's consciousness of, for example, an opera, can have parts which are auditory, parts which are visual, and parts which are linguistic. Radical separation of these parts by preventing commissural unification into a "seamless" whole is arguably best described as yielding part-consciousnesses. The point is supported by another consideration, that insofar as consciousness involves propositional attitudes specifically, and beliefs and knowledge, then truncating some of these substantially and systematically ought to leave at best a part-consciousness. In saying this, we are resisting the claim that consciousness is essentially linguistic (which would yield the conclusion that for commissurotomy subjects there is only one consciousness, the one we talk to). There is ample literature on nonverbal propositional attitudes and nonverbal mentality in general (e.g., Mortensen 1989); and, in any case, for commissurotomy there is plainly at least some sort of part-consciousness operating in the experimental conditions which test the capacities of the nonlinguistic side. If that isn't consciousness-like, what is?

6. Returning to persons, qualification has to be made in light of the fact that normal persons also have two person-like parts, but the parts are unified by the corpus callosum. Unless we want to say, as Puccetti does, that even normally there are two people inside your skull, it should be allowed that if the communication between the part-persons is sufficiently good, no matter what the precise means of communication is, there is only one person (consciousness, whatever). This seems to be the central point of disagreement between Puccetti and Dennett. Dennett emphasises the fleetingness of manifestation of an additional consciousness, the speed and success in finding strategies for dealing with everyday problems, the sheer normality of everyday life for the commissurotomised subject. He mocks the suggestion of a second, dumb, trapped, nonlinguistic person. Puccetti, on the other hand, is impressed by the drastic changes in capacities brought about by the commissurotomy, as revealed in the well-known experimental conditions. On these differences there is an obvious compromise: for such subjects, most of the time there is enough unity for it to count as one person or consciousness; while sometimes, particularly under artificial experimental conditions, the parts are forced apart into part-persons (but not whole persons), only to reunite when communication strategies are allowed to be re-established. An analogy is a group of hikers splitting into subgroups as they take separate paths, only to reunite into the original group when the paths reunite. The compromise is evidently more in keeping with Dennett than Puccetti (though see below, paragraph 10). Puccetti seems to be resisting such a possibility when he argues that one isn't a French speaker only occasionally, when one actually speaks it. But this is to assume that being a person is more like being a French speaker than like being a group, and we take it that Dennett's multiple-drafts idea is designed to resist just such an assumption.

7. Nonetheless, there is a substantive issue here: does the commissurotomised subject display enough unity in ordinary life for it to be reasonable to count a whole person, just one, on ordinary occasions? On this point, the authors of this commentary confess to being in some disagreement. Two of us [CM, GO] believe that the answer

 is yes. The other thinks that there has to be some way of registering the
weaker linkage vouchsafed by cross-cuing, and suggests that the notion of "part-unity" could be used. Counting does not always have a determinate application: how many clouds, one or two? This might be modelled in biological systems in terms of the closeness with which the symbiotic parts interact: the closer the interaction, the more fruitful it is to concentrate on the unified system as a whole, while the greater the dissociation, the less the parts overlap and the more we have two things interacting, the point being that for a partly unified system the question of one or two need not have a determinate answer.

8. We are more concerned, however, to argue that if the answer is two, then it is two part-persons and two part-consciousnesses that we are dealing with. This implies a view of the hemispherectomy case raised earlier: one part-person and part-consciousness but not more, we think. And, to (nearly) repeat an earlier point, there is no reason to assimilate the morality of killing such a subject with the morality of killing a decorticated subject. On the other hand, if the surviving part can learn to regain the lost functions then there would seem to be one whole person again. People and consciousnesses can grow too.

9. Dennett must take the blame for some of the confusion, however. As we noted before, he emphasises the unity of the parts, perhaps most strikingly in his Jekyll and Hyde argument: "We need conflict or strong difference to shake our natural assumption that to one body there corresponds at most one agent" (quoted by Puccetti, para. 7). But in other places it sounds more like our part-persons idea: "it isn't the case that commissurotomy leaves in its wake organizations both distinct and robust enough to support such a separate self" (Dennett 1991, p. 426), and "my theory says there isn't [such a right hemisphere self], and says why: the conditions for accumulating the sort of narrative richness (and independence) that constitutes a "fully-fledged" self are not present" (ibid). Note the tension between saying that there isn't enough distinctness for the parts to be separate selves, which suggests enough unity for a single whole person, while also saying that there isn't enough robustness within an individual part for a separate self, which suggests only a part-person. Again, insufficient independence can only be insufficient independence between the parts, thereby suggesting sufficient unity for a single whole person, whereas insufficient narrative richness suggests (separable) part-persons.

10. With these distinctions made, we trust that the issues have been appropriately clarified.


Dennett, D.C. (1991) Consciousness Explained. Boston: Little, Brown.

Mortensen, C. (1989) Mental Images: Should Cognitive Science Learn From Neurophysiology? in P. Slezack and W. Albury (eds) Computers, Brains and Minds. Dordrecht: Kluwer.

Puccetti, R. (1973) Brain Bisection and Personal Identity. British Journal for the Philosophy of Science 24: 339-55.

Puccetti, R. (1993) Dennett on the Split Brain. PSYCOLOQUY 4(52) split-brain.1.puccetti.

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