One of the commonest of daily experiences is that of recognizing our friends. A less common, though still fairly familiar, experience is the decision that a certain person is or is not the person he claims to be. The problem of personal identity is that of clarifying the principles behind these indispensable processes of reidentification. To reidentify someone is to say or imply that in spite of a lapse of time and the changes it may have wrought, the person before us now is the same as the person we knew before. When are we justified in saying such a thing, and when are we not?

The Basic Problems

Some philosophers have said that we are never justified, because sameness and change are, in themselves, incompatible. They have argued that it is almost paradoxical to say that something has changed and yet is still the same...