 The notion of a sortal overlaps with significant and long unresolved philosophical controversies in philosophy of language, metaphysics and philosophy of mind. Philosophers disagree about whether there are essences and those who agree that there are disagree about whether essences exist at a very general level—animal—or a specific level—tiger. And philosophers disagree about criteria of identity. Some believe that criteria of identity are relative to a sortal in ways that can conflict, two things might be the same ship but not the same wood. Others believe that identity conditions vary by sortal but do not conflict—that the criteria for the identity of ships is different from that for people but since people are not ships, the answers could not conflict. Others believe there is only one general identity criterion regardless of sortal, and others argue that there are no identity criteria. On one extreme view (Sider 1996), nothing exists at more than an instant. We, like most of the literature on sortals, will not take that view into account in what follows.