Colloquia Theologica Ottoniana

A theory of personal identity is a theory about what makes some person P2 at a time T2 the same person as some person P1 at an earlier time T1. Most contemporary theories are “complex theories”. Complex theories hold that the identity of two persons depends on a certain degree of “continuity” or “connectedness” between the two persons of one or more features – P2 having some of P1’s body or brain, and/or being able to remember some of the experiences of P1. All these theories are open to the arbitrariness objection – that any such theory has to state exactly what degree of the relevant feature would make P2 the same as P1, and the choice of any particular value for that degree would be entirely arbitrary. To meet this objection complex theories have to claim that being the same person as P1 is a matter of degree. But such “partial identity theories” are open to the objection that they have the consequence that more than one later person could be partly identical to P1, a consequence that cannot be spelled out coherently. It follows that either P2 is fully identical to P1, or P2 is not at all identical to P1, and so leads to the “simple theory” of personal identity, that personal identity cannot be analysed in terms of features of which there can be different degrees. But there must be a difference between a person at T2 who is P1 and one who is not P1. And so the paper concludes that P2 is identical to P1 if they both have the same indivisible non-physical part, that is the same soul; but otherwise they are not the same. What makes a person who they are is their soul.