Personal Identity

Some Terminology

The phrase personal identity means different things in philosophy, psychology, and everyday life. Our concern here is with the phrase as it has been understood by philosophers. Typically what philosophers who discuss personal identity want to know is what makes it the case that a person at one time is the same as a person at some later time. Though less frequently discussed, philosophers have also wanted to know under what conditions a single body houses one person or two (as in, e.g., cases of split personality).

Since the phrase personal identity derives from person and identity, these two notions are conceptually prior to that of personal identity. However, this doesn't mean that they are prior in every sense. Certainly, if we know what a person is, then we know what it is for the same person to persist through time. Still, it may be that the best way to discover the nature of persons is by sifting through competing theories of what it is for a person to persist through time. If we know what changes a person may or may not survive, we will have more idea of what kind of thing is a person. This is what motivates and justifies the methodology of thought experiments.

Some comments on identity and person: In this discussion, we mean identity in the sense of strict numerical identity, not in the sense of qualitative identity. So we are not concerned with the sense of [Page 583]identical as it appears in identical twins. Twins may be very similar (qualitatively identical), but numerically they are two people, not one. Note also that in talking of identity our concern is metaphysical not epistemic. That is, we are not asking about the kind of evidence we typically rely on in making judgments of personal identity (e.g., physical appearance). Our concern is with what, if anything, constitutes personal identity. What makes it the case that a person at one time is the same (numerically) as a person at some later time? The answer to this constitutive question will not be the same as the answer to the evidential question, since evidence such as appearance, or even fingerprints, is never a logical guarantee of personal identity.

What of the term person? Why do we have such a term? What distinctive work does it do? It would not be too controversial to maintain that we use person to delineate a certain kind of mental being—namely, a self-conscious mental being. This definition puts no restriction on the kind of entity that can be a person. As far as the definition goes, persons could be bodies, brains, nonphysical souls, robots, Martians, parrots, dolphins, or creatures yet to be encountered. This liberality is a strength but also a weakness. For one might have hoped that an answer to the question “What is a person?” would tell us what ontological category (or category of being) we belong to; that is, it would answer the question “What are we?”

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