  Reid holds that numerical identity is, strictly speaking, indefinable, but it can be contrasted with other relations, such as diversity, similarity and dissimilarity (Essays, 263). It requires a continued existence over time—a duration—and requires that there be no two beginnings of existence. Because mental states are fleeting and non-continuous they cannot remain identical over time. A mental state may be indistinguishable from a previous mental state, but because mental states do not have a continued existence, no mental state at one time can be numerically identical with another at a different time. As a result, persons cannot be identified with their thoughts, actions or feelings (Essays, 264). However, according to Reid, thoughts, actions, feelings and all other mental operations are had or performed by a subject that has a continued existence and that bears the same relation to all them. The subject is an immaterial substance that thinks, acts and feels. According to Reid, this substantial self has no parts—it is indivisible—which contributes to its resistance to reductive explanation. Reid appeals to Leibniz’s notion of a monad to describe the indivisibility of this immaterial, substantial self (Essays, 264).