 Some of those who reject relative identity nevertheless accept that a cannot be identical with b, unless there is a more specific answer to the question whether a is the same thing as b. Many philosophers distinguish between two kinds of concepts which are applicable to whatever can persist through time. One kind is illustrated by the concepts of gold (as opposed to a quantity or piece of gold), snow, or rain. In the case of any such concept F, there is no answer to the question: how many Fs are there? In contrast, in the case of other concepts such as the concept of a horse, a tall person, an artwork, or a statue there is an answer to the question: how many Fs are there? For example, though we may not know it, there is an answer to the question: how many statues are there? Concepts of this last kind are often referred to as sortal concepts. Those who take the view that the question whether a is the same as b is illegitimate, unless it is construed as elliptical for the question whether a is the same thing of such and such a kind as b typically also hold the following views. We should distinguish between two types of sortal concepts: phase and substance sortals. A phase sortal such as child, utensil or prize is a concept that something can cease to fall under without ceasing to exist. If, in contrast, something falls under a substance sortal, it must always do so. If a at some time t is the same as b at the same, or a different time t′, it must, say advocates of the view in question, be that, for some sortal S, a at t is the same S as b at t′. For example, if John's favorite artwork at t is identical with Sally's most valuable possession at t′, there must be a substance sortal, the concept of a statue as it may be, under which John's favorite artwork at t, and Sally's most valuable possession at t′ both fall. Moreover, a substance sortal is said to go together with a criterion of identity where a criterion of identity associated with substance sortal S is, inter alia, a criterion for some earlier S being the same S as some later S.