The Brain Transplant

Brownson

Shoemaker's case involves two men: Brown and Robinson. A surgeon removes both their brains and, through an accident, transplants Brown's brain into Robinson's body. The Robinson-bodied patient who wakes up, Shoemaker calls him "Brownson," has Brown's memories, Brown's personality, Brown's beliefs, and considers himself to be Brown. Robinson, brain-dead, does not wake up.

Most people's intuition is clear: Brownson is Brown. The body is Robinson's, but the person is Brown. If Brown had known what was about to happen, he would have been right to expect to wake up in Robinson's body. This intuition does real philosophical work: it suggests that the brain, not the body, is the carrier of personal identity.

What the brain carries

The argument is that personal identity tracks psychological continuity, and psychological continuity is implemented in the brain. Your memories, your personality, your patterns of reasoning and emotion, all of this is stored and processed neurologically. If the brain moves, psychological continuity moves with it. The rest of the body, from this view, is more like an organ the person uses than a component the person is.

This is a body theory in one sense, since it does require physical continuity of a particular physical object. But the relevant physical object is the brain, not the body as a whole. Shoemaker's case was designed to show that body-level continuity is too crude: the question is which physical system is continuous with which, and for persons, the brain is the decisive system.

The hard cases

The clean case gives a clean answer. The hard cases are more interesting. Suppose only half the brain is transplanted. Suppose both hemispheres are transplanted into two different bodies. The left hemisphere controls language and much of conscious experience; the right hemisphere controls spatial reasoning and other functions. Each recipient would have some but not all of Brown's psychological properties.

These cases connect directly to the fission problem: if both recipients have a roughly equal claim to psychological continuity with Brown, who is Brown? Standard identity theory requires a one-to-one relation, so they cannot both be Brown, but it's hard to see why either should have a stronger claim. Shoemaker's clean case opens the door to the messy ones, and the messy ones suggest that "who survives" may not always have a determinate answer.