 How general this makes its application is a matter of controversy. In particular, if persisting things are thought of as composed of (instantaneous) temporal parts (see discussion below), the problem of supplying a diachronic criterion of identity for persisting concrete objects can be regarded as the problem of providing a two-level criterion. But if persisting things are not thought of in this way then not all persisting things can be provided with two-level criteria. (Though some can. For example, it is quite plausible that the criterion of identity over time for persons should be thought of as given by a relation between bodies.)