  A theory of personal identity is intended to account for how a person remains identical over time. When analyzed in terms of items that are fleeting and non-continuous—ideas, memories, thoughts—identity is reduced to diversity; that is, it is eliminated. By contrast, if one locates personal identity in that which thinks and remembers, and which has a continued, uninterrupted existence, one purchases personal identity at the cost of admitting that the self is a substance. Reid captures Locke on the horns of a dilemma: either the self is a substance, in which case it remains identical over time, or the self is not a substance, in which case there is no personal identity. Reid holds that this dilemma applies with equal force against any reductionist account of personal identity that employs the theory of ideas, for example Hume’s bundle theory of the self (Essays, 473–474).