Williams's Body Swap

Two versions, one event

Williams describes a scenario in two ways. In Version A, you are told the following: your memories and personality will be transferred to body B, and body A (your current body) will receive body B's memories and personality. After the transfer, body B will be given a wonderful life, and body A will be tortured. Which body would you prefer to be? Most people say they'd prefer to be body B. They identify with whichever body ends up with their psychology.

In Version B, you are told simply: your body will be tortured tomorrow. Before the torture, your memories and personality will be altered completely. You will wake not remembering who you are. Then you will be tortured. How do you feel now? Most people report fear. They are afraid, even knowing their psychology will be changed first.

Both descriptions describe the same physical sequence: two bodies, a swap of psychological properties, one tortured, one given a good life.

Why the intuitions conflict

In Version A, you are given a third-person description of two bodies and asked to choose. You naturally identify with whichever body ends up with your psychology. In Version B, you are given a first-person description of what is going to happen to you, and the prospect of your body being tortured produces fear, even with the caveat about altered memories.

Williams's point is that these divergent intuitions reveal something. The psychological continuity theory, favored by Locke and later Parfit, says you go with your psychology. Version A supports this. But Version B suggests that bodily continuity has an independent grip on survival. When you are told "your body will be tortured," no amount of prior psychological alteration makes the fear disappear. The body is not merely a vessel you happen to be using.

Williams's conclusion

Williams argued that bodily continuity matters more for survival than psychological continuity theorists allow. Your body is not incidental to who you are. The fear response in Version B is not irrational; it tracks something real about what survival means from the inside.

This is a minority position in contemporary philosophy of personal identity, where psychological continuity views remain dominant. But Williams's argument has never been refuted, only absorbed. It shows that the question of personal identity is not settled by asking which theoretical account best systematizes our intuitions, because the intuitions themselves pull in different directions depending on how the scenario is described.