The issue of identity: Being self and other

What would it be like to be someone else, to be in another person's head? An intriguing question, for sure, which does not come with readily available answers but which certainly allows us to fantasize endlessly. A question such as this one has been food for thought for filmmakers, writers, and artists alike – and philosophers of course. In his 1690 treatise on personal identity, John Locke invites his readers to imagine the following. A prince and a cobbler, two men with completely different lives and appearances, decide to swap the contents of their memory with each other. This implies that the memory of the prince, including all his personal recollections, is stored in the body of the cobbler. All princely thoughts and recollections of a spoiled life as prince have now become part of the cobbler's body, marked as it is by a life of hard work. For Locke this thought-experiment gives rise to the question of what this particular alteration does to the identity of the prince. Is he still the same person?

Since Aristotle a distinction has been made between qualitative identity and numerical identity. We speak of qualitative identity when two things or persons have the same traits or qualities. For example, monozygotic twins are identical from a qualitative angle. But they are not numerically identical because they comprise two different individuals. Dr. Jekyll and Mr. Hyde, in contrast, quite differ from a qualitative angle, but numerically they are one because they comprise a single human being.

When posing the question of the identity of the prince, Locke was not concerned with qualitative identity. In a qualitative sense, after all, we change constantly in the course of our life: our body is subject to an ongoing process of transformation. It is also quite normal for particular character traits to change; a shy young person, for instance, may well conquer her shyness at a later stage of her life. In Locke's thought-experiment, the prince did not gradually change a little, but radically obtained an altogether different body and appearance – the body of some other person. Such intervention is not just a qualitative change; rather, it involves the fusion of two different individuals, and therefore we should ask if the prince has numerically remained identical.