Leibniz holds that we cannot give an account of the synchronic and diachronic identity of a person without appealing to a substance. This paper analyses his reasons for this anti-Lockean thesis. It first looks at his theory of substance, paying particular attention to his commitment to the Principle of Sufficient Reason: the existence of a well-ordered series of mental states cannot be sufficiently explained without reference to a substance. The paper then examines the distinction Leibniz draws between the substance as (...) the “real person” and the “appearing person” that comes into existence through reflexive consciousness. It argues that there can be no appearing person without a real person and looks at the relationship between these two types of person. Leibniz’s distinction is still relevant because it shows that questions concerning the metaphysical constitution of a person need to be carefully distinguished from questions concerning the psychological construction of a personality. (shrink)