 and it is well known that this axiom schema runs into trouble when ‘=’ is given a diachronic reading. (See the entries on  change  and  identity over time.)  The problem is a general one. Whatever the solution, it will therefore apply to the case at issue as well, and in this sense the above-mentioned objection to (28) can be disregarded. For example, the problem would dissolve immediately if the variables in (28) were taken to range over four-dimensional entities whose parts may extend in time as well as in space (Heller 1984, Lewis 1986b, Sider 2001), or if identity itself were construed as a contingent relation that may hold at some times or worlds but not at others (Gibbard 1975, Myro 1985, Gallois 1998). Alternatively, on a more traditional, three-dimensional conception of material objects, the problem of change is often accounted for by relativizing properties and relations to times, rewriting (ID) as