 By synchronic identity we mean an identity holding at a single time. By diachronic identity we mean an identity holding between something existing at one time and something existing at another. One question is whether synchronic and diachronic identity are different kinds of identity. Some philosophers are willing to countenance different kinds of identity. Others are reluctant to do so. One philosopher who is willing to postulate a multiplicity of different kinds of identity is Peter Geach. Geach, among others, has addressed puzzles about both synchronic and diachronic identity by denying that there is a single absolute relation of identity rather than a host of relative identity relations. On this view we cannot simply say that a is identical with b. Instead there must be a concept of a kind of thing, a so called sortal concept, that serves to answer the question: a is the same what as b? This is so, according to the champions of relative identity, because the following can happen: a and b both fall under the sortal concepts F and G, a is the same F as b, but a is not the same G as b (Geach 1967, and see the entry on  relative identity).