Identity of Persons
A standard topic in medieval philosophy was the search for a principle of individuation—that is, the question about what it is that makes an individual (object or person) the individual it is and distinguishes it from all other individuals of the same kind. Indeed, the medieval disputes formed a major part of the background for the early modern discussions about the issue. But from about the middle of the seventeenth century onward most philosophers (for example, Thomas Hobbes, Robert Boyle, Locke) neglected the issue of individuation and focused on identity over time instead—that is, on the question about the requirements for an individual's remaining the same over time, although that individual may have undergone some change. Also, there was a marked shift in the discussions away from a primarily ontological to a more subjective treatment of the topic. Our concepts of those things whose identity is in question came to be regarded as crucial for dealing with problems of individuation and identity. In Locke this move is connected to his view that we cannot know the real essences of substances. For Locke the question of how much a thing can change without losing its identity can be answered only with respect to "nominal essences"—that is, with respect to our abstract or "sortal" ideas of those beings whose identity is under consideration.
Early modern philosophers considered the issue of the identity of persons over time to be of special importance, as it is central to theological issues such as the doctrine of life after death and related moral questions. But for most of those thinkers (such as René Descartes) who believed that the soul is an immaterial substance, there was no real problem of personal identity at all. They argued that personal identity consists in the identity of a mental substance or soul and that the identity of the mental substance is a direct consequence of its immaterial nature; it is because of its immateriality that the mind is not subject to change and remains the same through time.
Locke's account marked a decisive break with both the Cartesian and Scholastic positions, which identified either the soul or the man (or human being) with the person as a res whose individuality and identity is constituted independently of and prior to consciousness. Locke treated the special problem of personal identity in accordance with his general theory of identity. Therefore, he argued that we need to be clear about the concept of person in order to be able to determine what constitutes the identity of persons. And to be clear about the concept of person, we have to distinguish it from those of thinking substance or spirit, and of man or human being because each of these concepts carries with it different identity-criteria. The identity of the self as man (or human being) consists in the identity of the same organic body. As we do not know the real nature of the soul as substance, personal identity is accounted for in terms of what we know about the self through inner experience or consciousness uniting thoughts and actions. To consider the self as a person is to consider the self with regard to all those thoughts and actions of which it is conscious. Through consciousness we link present with past thoughts and actions, thereby constituting our personal identity. Only with respect to our personal identity are we morally and legally responsible for past actions. This is why Locke said that "person" is a "forensic" term.
Locke's theory aroused controversy soon after its first publication and inspired critics and defenders throughout the eighteenth century. One standard objection was the charge of circularity, urging that consciousness presupposes personal identity and therefore cannot constitute personal identity (John Sergeant, Joseph Butler). However, this charge presupposes the very thing Locke challenged, namely that the person is an object, thing, or substance to which consciousness relates as to an already individuated being. Another standard objection was that Locke's theory is inconsistent with the transitivity of identity because consciousness is not transitive (George Berkeley, Thomas Reid). In Germany, Locke's most important contemporary critic, Gottfried Wilhelm Leibniz (1646–1716), distinguished between the metaphysical identity of the self (as immaterial substance), which is secured a priori by its intrinsic nature or "complete notion," and the moral identity of the self (as person), which is constituted by consciousness (Nouveaux Essais, 1704). However, while Locke argued for keeping personal and substantial identity separate, Leibniz maintained that the (personal) identity required for morality could be preserved only by the metaphysical identity of the self as immaterial soul. Through Christian von Wolff (1679–1754), Leibnizian theory dominated philosophy at German universities until about the middle of the eighteenth century.