Abstract

In this paper, I consider the validity and proper formulation of the only-x-and-y principle, which states, roughly, that whether a later individual, y, is numerically identical to an earlier individual, x, can depend only on facts about x and y and the relationships between them. In the course of my investigation, I distinguish between two classes of physical entities - those that exist in a 'real' sense, and those that exist in a mere Cambridge sense. This distinction is grounded in Peter Geach's distinction between 'real' and mere Cambridge change. I argue in favor of a modified version of the only-x-and-y principle - the qualified only-x-and-y principle - which applies to entities that exist in a 'real' sense, but not to mere Cambridge entities. It is also argued that the plausibility of the qualified only-x-and-y principle has more to do with facts about the nature of causality than with intuitions we have about existence or numerical identity. I finish by considering some traditional objections to the only-x-and-y principle, and conclude that they do not succeed in refuting the qualified only-x-and-y principle