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It is simply an undeniable fact that the human mind is linked to the human brain in very intimate ways. As cognitive sciences and neuroscience and all the related disciplines progress, we come to know more and more about what brain processes occur when we perform certain tasks or have certain experiences. The disturbing thing is that what we are finding out is that our conception of how our minds work on a day to day, subjective basis is probably about as wrong as you could ever imagine something could be. Our conceptions about how we believe and desire and think and write philosophical papers seem to imply some sort of language use, inside the brain as it were. But the brain doesn’t seem to function in a way that leads neuroscientists to believe that there are little sentences flying around in there.

Another thing that our oh-so-off-the-mark conceptions about the workings of our minds have probably gotten completely wrong is the idea of the unity of consciousness. As I understand the idea, it’s really quite intuitive. To ourselves we seem to have one stream of consciousness and one set of memories and one set of cognitive abilities each, all of which constitutes (respectively) our one mind. Doubt looms in our minds however as a result of research into the phenomenon of those people who have had their corpus callosum severed. The corpus callosum (for the brain anatomy uninformed) is that part of the brain that is the only direct connection between the right and left hemispheres of the brain. These people, whose two hemispheres of brain cannot communicate directly, display indications under experimental conditions that they are not always aware of the same things at the same time with both of their hemispheres. And yet they function normally when they interact with other people on a casual basis. Under normal circumstances they act as if they had one mind just like the people who haven’t had their corpi callosi (corpuses collosums?) severed. Thomas Nagel seems to think that the strange two minds/one mind problem posed by the situation I have sketched above goes to show that the notion of “the unity of a person” is useless as a scientific concept. And while we may never get past conceiving of ourselves as having single minds (or as having beliefs, etc.) we may have to give up searching for a physical basis that resembles our present conception.

To keep things from getting overly long and flowery, I’m going to try to state Nagel’s argument as succinctly as possible. After that I’d like to find some ways to avoid his conclusion that our conception of ourselves as single persons is a quaint illusion and not something real in the world. Single persons and the single minds that we each have aren’t illusions. My conviction is that these are real things. I intend to show that Nagel’s arguments are not adequate to show that they aren’t real. I don’t intend to prove that they are real: I don’t think that I could pass an accurate judgement on this issue even if I had all the relevant technical knowledge. Hopefully though, I can meet the challenge of Nagel’s arguments and show that we don’t have as much reason to doubt the unity of consciousness as he would like us to believe.

Let me start with just a sample of the background information that Nagel provides as to just what the ‘split brain phenomenon’ is. The “large transverse band of nerve fibers called the corpus callosum…play[s] an essential role in the ordinary integration of function between the hemisphere [sic] of normal persons.” (NM, pg. 433) This fact was found out only after people and animals had undergone surgery to sever this band of nerve fibers. Experiments with cats were performed, some of whom had had their corpis collosis (my Latin is very rusty) severed, and some of whom had not. Both groups also suffered the removal of the connections that bind each individual eye to both hemispheres of the brain (their optic chiasma nerves were severed). In this way, each eye was only able to communicate with the half of the brain that was on the same side of the body as the eye. The cats were then trained to do “simple tasks using one eye” in order to “see what happened when one made them use the other eye instead.” (NM, pg. 433) Those who had their corpus callosum intact could perform pretty well on each task regardless of which eye they could use. Those who did not have an intact corpus callosum however showed evidence that the left brain didn’t know what the right brain was doing. Under non-experimental conditions however, all the cats behaved normally that is, they didn’t suffer “serious deficits of behavior.” (NM, pg. 433)

Though no humans suffered the severing of their optic chiasma nerves (which would only have been fair to the cats), there are those who, because of severe epilepsy, underwent the surgery to sever their corpus callosum. The results of experiments done with the full consent of members of this extraordinary class of people revealed similar results. Experimenters had to be very canny to be able to isolate the reactions of the separated hemispheres, but having done that, they found that split brain patients displayed strong indications that the two hemispheres of the brain functioned almost completely independently of one another. Allow me to quote a rather lengthy passage that will provide many more examples of this independent functioning of the hemispheres than a crude paraphrase could:

The split brain patient cannot tell whether shapes flashed to the two half visual fields or held out of sight in the two hands are the same or different-even if he is asked to indicate the answer by nodding or shaking his head (responses available to both hemispheres). The subject cannot distinguish a continuous from a discontinuous line flashed across both halves of the visual field, if the break comes in the middle. Nor can he tell whether two lines meet at an angle, if the joint is in the middle. Nor can he tell whether two spots in opposite half-fields are the same or different in color-though he can do all these things if the images to be compared fall within a single half field. On the whole the right hemisphere does better at spatial relations tests, but is almost incapable of calculation. It appears susceptible to emotion, however. For example, if a photograph of a naked woman is flashed to the left half field of a male patient, he will grin broadly and perhaps blush, without being able to say what has pleased him, though he may say, “Wow, that’s quite a machine you’ve got there”. (NM, pg. 434)

So do these people have two minds or one? I’m of two minds on the subject. But seriously, Nagel thinks it’s quite ambiguous and for now I’d like to humor him. Under experimental conditions when the patients are stimulated in ways that elicit responses from isolated hemispheres, they seem to have two disconnected minds. But under normal circumstances, the same patients are for all intents and purposes completely indistinguishable from other, ordinary people. After considering split brain patients, Nagel wants to conclude “that the ordinary conception of a single, countable mind cannot be applied to them at all, and that there is no number of such minds that they possess, though they certainly engage in mental activity.” (NM, pg. 435) And furthermore, since we can’t supply an exact number of minds for these patients, neither can we supply one for ordinary people. I’m not going to claim that the possession of a unitary consciousness is essential to cognition, but I would like to show that there is a number of countable minds, namely exactly one countable mind. Even for split brain patients.

Nagel claims (in the right-hand column of pg. 437) that we ordinarily assume that only single minds have the ability to do all the things that the split brain patients couldn’t do in the quoted passage above. Perhaps this assumption is unfounded but given that assumption, the split brain patients couldn’t possibly have just one mind. They can’t do those things. They aren’t always aware of all the relations between mental events which occur at the same time or one right after the other when they cross the interhemispheric gap. Presumably this means that they have two minds. But they don’t act that way most of the time. It’s only when the reactions of the individual hemispheres are isolated that anomalies show up. So we conclude that they have one mind.

Oh, the tangled web we weave! Trading off the uncertainty that we all have about whether these patients have one mind or two, Nagel concludes that they don’t have any ‘whole number of minds’. The notion of countable minds is meaningless and doesn’t apply. And if it doesn’t apply in the case of these people, then it doesn’t apply to ordinary people either. This is because if Nagel is right that “there is no whole number of individual minds that these patients can be said to have, then the attribution of conscious, significant mental activity does not require the existence of a single mental subject.” (NM, pg. 438) A further point that Nagel makes is that the concept of a unitary consciousness is used to explain the “unified operation of…[the] components and functions [of the mind]”. (NM, pg. 439) If there is no such unified operation, then we are hard pressed to find a way to construe the mind as singular. Therefore, the idea of a single mental subject is (possibly) not essential to the existence of mentality in general. And because the idea is not essential, we may have to just do away with it if we ever expect to have a complete picture of how the mind works. Here is Nagel’s argument as best as I can construct it:

Premise 1: “One mind” doesn’t apply to split brain individuals.
Premise 2: “Two minds” or “x minds” doesn’t apply to split brain individuals.
Premise 3: So “conscious, significant mental activity does not require the existence of a single mental subject” Therefore, the idea of a single mental subject is (possibly) not essential to the existence of mentality in general.

Now it is incumbent on me to show how this line of thinking is mistaken and establish that even split brain patients probably have just one mind. My first point regards the question that Nagel poses to which his conclusion is the answer. This should already be obvious but for clarity’s sake the question is “How many minds do split brain patients have?” I think that this is the wrong question and that it leads the discussion in a contrived direction. A more appropriate question, in my humble opinion, is “What kind of mind do split brain patients have?” My contention is that split brain patients have a different quality of mind from ordinary people but that they have the same quantity of minds as everyone else in the world.

Basically this approach attacks premises one and two because I’m questioning whether we need to count minds. Nagel tries to apply the concept of ‘countable minds’ to the set of phenomena exhibited by split brain patients. I don’t think this concept works in just the way that he thinks it does. The way I construe the idea of a countable mind, split brain patients have as much right to that description as everyone else does. They think of themselves as one person, everyone who knows them thinks of them as one person, they act like one person, and even under experimental conditions they act in a unified manner at least in so far as they relate to the experimenter. Split brain patients merely have the unusual potential to have the reactions of their individual brain hemispheres isolated. They experience frustration when they can’t quite figure out what the other half of their brain is doing. The evidence seems to weigh much more heavily in favor of a single mind with altered functioning than in favor of an ambiguous number of minds.

The quality of the cognition of split brain patients could be understood as disjointed and odd from the point of view of those of us who have an intact corpus callosum. It isn’t necessary to call into question how many minds the person has. The personality of the person is intact and under most circumstances they can think and function normally. So this leads me to conclude that the unity of consciousness is a much more robust concept than Nagel seems to think. The unity of consciousness seems to survive even the severing of the direct connection between the hemispheres of the brain. Conscious, significant mental activity occurs not because of or only when there is unity of consciousness but the brain has a remarkable ability to preserve the unity of consciousness even when major parts of it are missing or damaged. Indeed there must be many cases of people who suffered losses of large or small parts of their brains, who (though they lose some functions and exhibit some odd behavior) continue to act and think as one person. Just because there is a certain amount of redundancy in the brain so that several parts can perform the same function doesn’t mean that there is no determinate number of minds present. I say there is one mind present as long as the person whose mind it is experiences it that way and others around him or her also experience them that way.

Now I may be coming across as simply giving my own, uninformed opinions about the nature of the split brain phenomenon and what makes a mind a unified thing. But it is exactly the fact that I can put my own equally plausible interpretation to the precisely the same set of data that Nagel does that shows that his argument doesn’t work. What comes clear out of all this is really just that our conceptions of what it takes to be a single mind are ambiguous. If my interpretation of the phenomena displayed by split brain patients as having a different quality of mind but not a different quantity really is just as plausible as Nagel’s account of an ambiguous and therefore non-existent quantity of minds, then Nagel’s account is just as questionable as mine.

The best next step for us to take is to make a good definition of what it takes for a person to have just one mind and what sorts of anomalies are allowed before this can come into question. I fear that the way Nagel would define the unity of consciousness would be different from mine in important ways and we’d have to debate for some time before we came to a conception acceptable to both of us. He would define it in terms of the unity of the functional elements of the brain itself and by the level of its connectivity and integration. I would define it in terms of the subjective experience of the person involved and by their behavior. In this way we see why Nagel thinks that no number applies to the ‘number of minds’ a person has since even those of us who have an intact corpus callosum don’t always have a perfect unity of the functional elements of our thinking. But by my conception of a unified mind only someone with severe multiple personality disorder could be said to have several minds. Split brain patients are merely curiously handicapped. At any rate, it should be clear that the split brain phenomenon does not unequivocally “show that our ordinary concept of what is a single person might someday be considered a quaint illusion.”