Is it possible to exist as two persons?
In his essay, “Personal Identity,” Derek Parfit challenges the conventional association between personal identity and numerical sameness. According to the perspective of numerical sameness, I am the same person I was yesterday, because on each day, there was exactly one person who was me, and for every moment in between, there was also exactly one person who was me. While Parfit admits that numerical sameness is often practically tied to personal identity, he does not believe it is a necessary condition for personal identity in all possible worlds. In “Personal Identity,” Parfit argues that one person can survive as two successor persons, in a radical departure from numerical sameness perspective. Parfit’s argument rests on two assumptions. The first is that a person can survive a brain transplant, meaning, if my brain were taken out of my body and put into a new (brainless) body, the resulting person would, to some degree, be me. (Parfit, PI, 15) The second assumption is that a person can survive with half a brain. Parfit combines these assumptions to prove that I could survive as two successor persons, using a futuristic medical experiment as an example. If my brain was split in half, and each half was transplanted into an empty body, I would survive as both people. There is no reason to doubt the truthfulness of Parfit’s second assumption. People have survived after suffering an injury which destroys half of their brain. However, we do not have good reason to believe that I could survive the brain transplant (to any degree).
In order to contextualize my answer to the question, it is necessary to examine Parfit’s conception of the word “survival.” Parfit uses the word to imply psychological continuity. (Parfit, PI, 20) I survive the brain transplant because my psychological experience in the new body resembles my psychological experience in my old body, to a certain degree. But Parfit argues that my experience will be psychologically continuous during the operation itself. In what sense, when my brain is isolated in space, between bodies, is my experience psychologically continuous?
As a matter of practicality, it does not seem at all possible that a brain could survive while entirely detached from the body, and vice versa. But assuming scientists have synthesized some equivalent of cerebrospinal fluid, and my brain cells are able to survive, it is still unclear whether I could be reduced to a mass of cells in this state. During the transplant, I would not be able to see, feel, hear, taste, smell, or breathe. My experience would be utterly devoid of perception. I would be limited to a state of existence far less substantial than that of a coma patient, and only nominally more substantial than the state of being dead. There is nothing obvious about calling a transition to this state of existence a case of psychological continuity, let alone a surviving person. After all, such an existence is characterized by a total lack of perception. If all perceptual activity subsides, just what part the psychological experience is continuous?
This question represents what I will call the “continuation problem” which faces Parfit’s transplant example. But, assuming we are charitable, and set this problem aside, we are now faced with the problem of “integration.” Perhaps, like Parfit, we agree that I can survive without my body. This does not necessarily imply that I can survive with a new one. As the surgeons attach my brain to the nervous system of the new body, it would become stimulated by an entirely distinct set of perceptions. The curvature of the eyes in my new body would alter my perception of light, the shape of my new ears would change my perception of sound, variations in weight, height, and center of mass would change my perception of feeling, and so on. In order for me to survive, I would need to have some memories of my prior experiences (which would likely entail remembering the perceptions of my old body), but these memories would not align with my current experience. There is no clear way my brain could go about reconciling this difference in information. If there are no perceptual similarities between the old psychological experience and the new one, it is unclear how these experiences can be regarded as continuous.
In arguing that a person can be reduced to a brain, Parfit will have to provide solutions to the problems identified above. This seems a hopeless task, so it is useful to reevaluate Parfit’s initial assumption. It is not obvious that a person can survive as a mere brain because a person is more than a brain; a person is an interdependent relationship of brain and body. The body provides the brain with sensory information, and the brain provides the body with regulatory information. Together, these related parts form a person, but when one element is missing, the person does not survive. Parfit was aware of one half of this relationship (after all, he does not argue that a person can survive without a brain), but he failed to notice its reciprocality. In his case of the transplant, the relationship is severed, and then created anew. Therefore, the only conclusion that can be made about Parfit’s transplant example is that my brain cells are recycled and survive, but whether I survive remains unclear.
Sources
The Philosophical Review, Vol. 80, No. 1. (Jan., 1971), pp. 3-27.