The problem of personal identity in the form in which it is so widely discussed today had its origin in the late seventeenth century, in John Locke's chapter ‘Of Identity and Diversity’ which he added to the second edition of his Essay concerning Human Understanding (1694). That chapter contains the most detailed and original contemporary treatment of the problem, challenging traditional views about both personality and identity. It was, indeed, revolutionary, and some aspects of it are still much discussed by philosophers. Locke was not, however, the only seventeenth-century philosopher to consider the topic seriously and at length. Problems of personal identity and of identity in general were widely debated long before the seventeenth century, in relation not only to metaphysics, or what is now called ‘philosophy of mind’, but also to moral, legal, and, especially, theological questions. The problem of identity and individuation in general – that is, the problem of what constitutes the identity of any object – is discussed in Chapter 9 of the present book. That problem is the historical as well as the systematic basis for the question of what constitutes the identity of persons. But there have been various responses to this latter question, depending not only on views of identity but also on which concept of person is applied. Indeed, from the notion of person adopted by some philosophers a genuine problem about the identity of persons might not even arise.