Exceptional persons: on the limits of imaginary cases

Ever since Locke (and particularly in the last 50 years or so) the philosophical literature on personal identity has centred on arguments of a certain type. These arguments use an assumed convergence of response to purely imaginary cases to defend revisionary conclusions about common-sense beliefs concerning the nature or importance of personal identity. So, for instance, one is asked to contemplate a case in which A's brain is transplanted into B's body, or a case in which some of C's memories are implanted in D's brain, or a case in which information about the arrangement of the molecules which compose E is used to create an exact replica of E at another point in space-time.

Thinking about these cases is supposed to help us tease apart the relative roles played by features that coincide in all (or almost all) actual cases, but which seem to be conceptually distinguishable. So, for instance, even though we can ordinarily assume that the beliefs, desires, memories, etc. which are associated with a given body will not come to be associated with another body, it does not seem to be in principle impossible that such a state of affairs should come about. Indeed, it seems that we can describe a mechanism by which such a situation might come about: for instance, A's brain (and with it A's beliefs, desires and memories) might be transplanted into B's body. And since the scenario described strikes us as something of which we can make sense, it seems we can make judgments of fact or value about which of the two factors really matters in making A who she is. We might ask, for instance, whether it would be true to say that A had survived in a body that used to belong to B, or whether it would be right to punish the B-bodied human being for A's actions, or whether if we were A before the intended operation, we ought to worry about what would be happening to the B-bodied person afterwards. And on the basis of these judgments about what we would say in the imaginary case, we can return to the actual case having learned something about which features are essential and which accidental to our judgments concerning the nature or value of personal identity. My goal in this paper is to suggest reasons for thinking that this methodology may be less reliable than its proponents take it to be, for interesting and systematic reasons.