5.3 Indeterminacy in the Number of Persons

11. The hypothesis that it is indeterminate how many per­sons a split-brain subject is comes from Nagel (1971). According to Nagel:

We assume that a single mind has sufficiently immediate access to its conscious states so that, for elements of experience . . ., the mind which is their subject can also experience the simpler relations between them if it attends to the matter. Thus, we assume that when a single person has two visual impressions, he can usually also experience the sameness and difference of their coloration, shape, size, the relation of their position and movement within his visual field, and so forth. The same can be said of inter-modal con­nections. The experiences of a single person are thought to take place in an experientially connected domain, so that relations among experiences can be substantially captured in experiences of those relations.

Split brain patients fail dramatically to conform to these assump­tions in experimental situations, and they fail over the simplest matters. Moreover, the dissociation holds between two classes of conscious states each characterized by significant internal coher­ence. (1971, p. 407)

Nagel continues:

These considerations lead us back to the hypothesis that the patients have two minds each. (1971, p. 408)

But this is untenable, according to Nagel, given the overall behavioral integration exhibited by split-brain patients. The upshot supposedly is that there is no whole number of minds these patients have! And this, in turn, should lead us, Nagel thinks, to be skeptical about the concept of a single subject of consciousness.

(13) I am unconvinced. Recall the case I imagined in chapter 3 of the person, buried in a horizontal position in sand, whose eyes and ears are moved away from his body above the sand, while the relevant neural connections are stretched but remain intact. This person is put in this state while uncon­scious and wakens to have an audiovisual experience of himself on a roller coaster (as presented to him by his eyes and ears in an IMAX theater) while simultaneously having tactual and bodily experiences of being motionless, sur­rounded by sand. He has an experience of something A and an experience of something B, but he cannot connect A to B in his experience. It is as if he is in two spatially disjoint worlds. He wonders, of course, how this could be happen­ing. In so doing, he wonders how he could be experiencing both these things. "What has happened to me?" he asks himself. "How could I be experiencing the things I am?" Perhaps, if he is clever enough, he can work out what must have happened. Be that as it may, there seems no question here that there is a single subject. And this subject does not conform to Nagel's assumptions.

14. There are other cases. People who have impairments in their ability to visually recognize spatial relations between things often have corresponding impairments in imagery. They are able to draw or describe the visual appearances of things in scenes no longer present, for example, a chair, a sofa, a lamp, a clock in a familiar sitting room, but they do poorly at drawing or describing the spatial arrangements of those things.² For these people, it is as if, when they view the scene originally, experiences are generated of the different items in it without there being any conscious experience of their spatial relations.

15. One might respond that in both the above cases the sub­jects experience certain spatially unconnected items together. So, there is at least an experience of the relation of unity or togetherness. This is not clear, however. Take the example of the people with object-centered "tunnel vision." These people are blocked from any interobject comparisons. As they focus on one object, they lose cognitive awareness of the other objects. It certainly seems conceivable that this cognitive deficit is mirrored at the level of the experiences themselves on which the cognitive responses are based so that there is no experience of different things together. And extending the example in this way surely should not lead us to the conclu­sion that two persons are present undergoing the experiences!

Still, Nagel is right to say that, for split-brain patients, there are two classes of mental states characterized by an internal coherence, at least if he has in mind attitudes and experiences present while the experimental controls are still in play. But, as emphasized earlier, these two classes are not like those undergone by individuals with a multiple person­ality disorder who really are two different persons. Split- brain patients do not both believe that P and not believe that P, want that P and not want that P, experience that P and not experience that P.

(16) Nagel also comments that we have no clear idea of what it is like for split-brain patients, and further that we can­not project ourselves into their mental lives. But this, I sug­gest, is confused. There is nothing it is like to be in the state the split-brain subject, S, is in, as he views the words 'pen' and 'knife' flashed up onto the left and right sides of the screen before his eyes. For insofar as there is any such thing as the state S is in at the given time, it is a conjunctive state, namely, the state of experiencing 'pen' and experiencing 'knife'. That state is not itself an experience, even though its conjunctive components are. So, there is nothing it is like for S to undergo the conjunctive state and, generalizing, noth­ing special it is like for split-brain subjects. Of course, there is something it is like for S to experience 'pen', and further there is something it is like for S to experience 'knife'. But what these states are like is just what they are like for any­one else. So, in one sense, we can project ourselves into the mental lives of split-brain patients: we know just what it is like to experience each experience a split-brain subject undergoes. What we cannot do is to imagine ourselves undergoing both the visual experiences a split-brain subject experiences at any given moment while the experimental controls are in place and simultaneously being aware, all in one introspective act, of the content of each. In this respect,

we are no different from the split-brain patient; for he can­not imagine that either.

17. Thus, I claim, where S is the split-brain subject, the fol­lowing are true:

S has an experience of 'pen'; S has an experience of 'knife'; S does not have an experience of 'pen knife'; S can be aware via introspection that he is having an experi­ence of 'pen';

S can be aware via introspection that he is having an experi­ence of 'knife';

S cannot be aware via introspection that he is having an experience of 'pen knife' (for S has no such experience); S cannot be aware via introspection (in a single act) that he is both having an experience of 'pen' and an experience of 'knife'.

If this is correct, then there is no special mystery about the mental lives of split-brain patients.