 What sort of bonding relationship holds among the things that compose material objects? Any answer to this question will be controversial. Let us set it aside, and move on to some further assumptions about the composition of nonliving composite material objects, namely that a great many of them persist for a while (some persist for a very long time) and that what composes them at one time normally differs from what composes them at other times. Exactly how this works is a complicated matter, but among the conditions that such objects must meet if they are to persist is that any change in their composition be incremental. (Even this condition is controversial. For more on material objects, see the article Material Constitution and Ordinary objects.) Consider the boulder again. Suppose that at one time, t0, it is composed of some molecules, and that all or most of these molecules remain bonded to each other until a later time t1. Suppose, too, that no or few (few as compared to the number of molecules that composed the boulder at t0) molecules come to be newly bonded to these by the time that t1 rolls around. Under these conditions the boulder undergoes an incremental change in composition, and it seems plausible to say that the boulder remains in existence over the interval t0–t1, and, at t1, is composed of the molecules that remain bonded together with the molecules that are newly attached to them. Presumably, it will also survive a series of such incremental changes in composition. But it will not survive drastic and sudden changes. It would stop existing, for example, if the molecules that compose it were suddenly dispersed.