  But there is a secondary, more subtle line of disagreement between Reid and Locke. Much of Locke’s chapter Identity and Diversity is dedicated to establishing that the self is not a substance, material or immaterial. By contrast, Reid holds that the self is a simple, unanalyzable immaterial substance with active powers. Reid argues that Locke cannot sustain both the thesis that the self is not a substance and the thesis that self remains identical over time. While Reid’s criticisms of the Memory Theory are more well known, his criticism of Locke’s insistence that the self is not a substance reveals two very different accounts of the metaphysics of identity. While Locke argues that the identity conditions for different kinds of things differ, so that the conditions under which a mass of matter, and an animal, and a person are not the same, Reid holds that identity is confined solely to substances that have a continued, uninterrupted existence and which do not have parts. In other words, according to Reid, strictly speaking the only real identity is personal identity (Essays, 266–267). “The identity…we ascribe to bodies, whether natural or artificial, is not perfect identity; it is rather something which, for the conveniency of speech, we call identity” (Essays, 266).