Two Heads
May be Better Than One, But How Else are They Different?
The most fundamental question one might ask about personal identity is what actually comprises a person? The most natural answer, perhaps, is that a person is his or her mind. Those who disagree with this statement would most likely argue that mere possession of a mind of some sort is not sufficient for a thing to be a person, rather than that a thing can be a person without possessing anything we would call a mind, so a mind is at least necessary for personhood. While it is unknown whether anything nonhuman can possess a mind, it is clear that for human beings, the mind exists as the brain or as a part thereof. The commonsense view is that there is a one-to-one correspondence between human persons and human brains. However, Thomas Nagel, in “Brain Bisection and the Unity of Consciousness”, disagrees that this commonsense view is the correct one, and discusses brain bisection experiments to support his claim that the number of persons or minds in a single brain is nothing as simple as one or two or any other particular whole number.[1]
He argues that there can’t quite be a single mind in a bisected brain, because experimenters can construct situations in which one such brain apparently has two conflicting centers of volition. How can a single mind, he asks, be in control of a body one side of which believes it held an unseen pipe and the other side of which believes it held nothing at all? On the other hand, how can there be two separate minds in control of a body whose actions normally appear so perfectly coordinated as to be indistinguishable from the actions of the paradigmatic example of a unitary consciousness: a person with an intact brain? What may seem the most attractive answer, that there is generally a single mind, which happens to split under certain experimental conditions, Nagel counters by pointing out that there is nothing significant enough about the experimental conditions for them to reasonably bring about something as significant as the dividing of one mind into two. Furthermore, not all of a subject’s actions during an experiment are completely disjoint, so there cannot be a total split in consciousness under experimental conditions. Nagel goes on to point out that the only difference between people with bisected brains and the rest of us is that most of us have brain hemispheres able to communicate more efficiently with each other through the corpus callosum, which is severed when the brain is bisected. If a patient without the corpus callosum can’t have one or two or any other particular number of minds, there’s no good reason for saying that the rest of us have any particular number of minds in our brains, either.
If we imagine medical technology were more advanced than it currently is, we can ask about a bisected brain transplanted to two new bodies the same question Nagel asks about a bisected brain in a single body. That is, how many persons would there be if the bisection of a brain were followed by the successful transplant of each hemisphere into a new body?[2] Leaving open for the time being Derek Parfit’s question, in “Personal Identity”, of whether the original person can be said to survive this operation, we should still uncontroversially be able to reject the claim that not even one person exists following the bisection and dual transplantation. Both bodies into which the hemispheres are transplanted would likely object adamantly to the notion that neither of them was a person. And as Parfit points out, were only one hemisphere to be transplanted and the other destroyed, we would not hesitate to say there was still a person (and furthermore, he argues, this is the same person who possessed the brain that was bisected in the first place), so why should we believe the number of persons is reduced if not one but both hemispheres are successfully transplanted into new bodies?
If we believe that only one of the recipient bodies, say that of the left hemisphere, is a person, we would have to conclude that were the left hemisphere destroyed instead of being transplanted into that body, no person at all would come out of the operation. And this conclusion would further force us to conclude either that someone who loses the functioning of their left hemisphere due to a stroke or injury is not a person, or that the successful transplantation of a full brain from the body of a person to another body results not in a person but in some kind of mindless automaton. The first conclusion conflicts with the way we actually view survivors of stroke or head trauma, and the second, for most of us, conflicts with our intuition in thought experiments involving intact brain transplantations. Supposing no telepathic abilities resulted from this operation, it seems we can also reject the claim that there remains one single person somehow spread between the two mindful bodies. Any doubts about the unity of a consciousness in one body with two apparent volitional centers must be compounded greatly if those centers now occupy distinct bodies and can act on conflicting volitions without any necessary interference from the other body. Further evidence against the unity of the resulting person(s) comes from what the two bodies themselves are likely to believe. If I were to ask Sinister, the body into which the left hemisphere had been transplanted, how he felt about my plan to reward him and to torture Dexter, the body into which the right hemisphere had been transplanted, he would likely answer very differently than if I asked him how he felt about my plan to draw and quarter Sinister and give a million dollars to Dexter. This difference of opinion would be irrational if the two bodies were really one and the same person, and yet we find it to be entirely reasonable.
So it seems we must conclude that there are at least two persons now that one brain has been divided between two bodies. But if we agree both that the same person comes out of a hemisphere-destroying stroke as went into it, and that the same person resides in a new body if that body successfully receives the transplant of the brain from his or her original body, then this solution has its problems, as well. If only Dexter survives the transplant operation, he would without a doubt believe he was identical with Ambi D’Exter, the single person who went into the experiment. The same, of course, goes for Sinister, were his body the only survivor of the transplant operation. We observers would have no reason to disagree with either of these beliefs. But by whatever logic allows us to say unproblematically that whichever single body survives the operation is run by the same person who went into the operation, we should be able to say unproblematically, if both new bodies survive, that they are both run by a person identical with the one who went into the operation. Unfortunately, if we still agree that Sinister and Dexter are not one and the same person, the transitivity of identity prevents us from consistently claiming that both are identical to some third person, namely Ambi, for to claim this would lead to contradiction.
The only option still left open appears to be that we cannot conclude, from whatever logic allows us to say Ambi survives either possible operation in which only one of the two new bodies survives, that if both bodies survive, they are both run by a person identical with Mr. D’Exter. This means that, if Ambi survives at all, he can only survive in one of the two bodies. As Parfit convincingly argues, it makes little sense to claim that while he would survive if only one body survived, he somehow would not if both bodies survived, because how could a double success mean a failure? And if he only survives as one of the two bodies, there can be no fact of the matter as to which one it is, since there is no reasonable criterion for determining which one that would be. Yet it is usually our inclination to believe that the question of survival is something that always has a factual answer, even if our imperfect knowledge may at times prevent us from ever knowing that answer. While some parts of his argument may be unconvincing, I believe we should concede to Parfit that survival is, at least in some situations, a matter of degree rather than an all-or-nothing question. If both bodies survive the operation, it may make the most sense to say Ambi D’Exter “survives at a rate of 50%” in both Sinister and Dexter. When only one body survives, Ambi still only survives at a rate of 50% in that body, but since that is an immensely greater rate than his survival in any other body, we can unproblematically talk as if Ambi survives fully. Similarly, if a person loses the use of one hemisphere due to stroke or injury, he or she may really only survive to some degree in the body controlled by the surviving hemisphere, but we can talk as if he or she survived fully, since there is no survival to any degree in another body.
What does this mean for the number of persons resulting from our hypothetical operation? As in the case of survival, what we say may depend more on contextual information than we would have thought initially. If we are talking about persons as they relate to any period after the operation, it seems we can talk unproblematically as if Sinister and Dexter are two persons, since they can at present have two distinct sets of memories of experiences since the operation and distinct sets of desires and intentions which will in the future play out as two separate courses of events. If we are talking about persons as they relate to any time in the past more distant than the operation itself, there should be no problem speaking as though Sinister and Dexter constitute a single person, because all of their experiences of the time period in question coincide perfectly. Problems arise when we wish to talk about some time period containing instants both before and after the operation (or when, over the course of a typical conversation, we wish to discuss times both pre-operation and in the future). The best way to deal with these problems, I believe, is to simply admit that we cannot unproblematically say anything in general, using current definitions, about the number of persons there are after the successful completion of our hypothetical operation. If medical technology advances enough to make this thought experiment a reality, and we still wish to talk about Sinister and Dexter as persons (as we surely will, if for nothing other than legal reasons), it seems we will have to revise our understanding of the concept of personhood (at least in a legal sense) to something that allows for consistent reasoning about the circumstances of such experiments. Until then, however, there should be few if any problems that arise from our continuing to talk about persons as if each has exactly one brain and each brain is possessed by at most one person.
[1] Perhaps the number of persons in a single brain is, in fact, Julius Caesar.
[2] Let us suppose, purely for the sake of simplicity, either that the original person was “ambidextrous” enough for both hemispheres to have equal verbal, spatial, and other capabilities, or that some medical technique has been developed that greatly accelerates the forming of new neural connections within the brain, so that its natural plasticity quickly results, after the operation, in two bodies with roughly equal abilities, even if the original person was not sufficiently ambidextrous.