 As noted in the last section, another source of apparent disunity in the concept of a criterion of identity is the distinction made between synchronic criteria of identity and diachronic criteria of identity. Criteria of identity can be employed synchronically, as in the examples just given, to determine whether two coexistent objects are parts of the same object of a sort, or diachronically, to determine identity over time. But as Lowe notes (2012: 137), it is an error to suppose that diachronic identity and synchronic identity are different kinds of identity and so demand different kinds of identity criteria. What then is a criterion of identity over time?