 Another direction in which to take the thesis that death is not amenable to classical definition is to argue that death is a natural kind whose essence may be obscure. Kripke influentially argued that natural kinds—kinds determined by nature rather than by human thinking, language, or interests—often resist adequate definition because their essential features may be entirely unknown to those referring to the kind in question (Kripke 1970). To define a term by reference to the features people originally used to pick out the kind in question won't do, because those features may be accidental, not essential, and speakers may even be mistaken about them. Those naming the kind whale might have thought whales were the largest fish in the ocean, but whales are not fish and their size relative to other creatures is a contingent matter. We can refer meaningfully to whales, to the creatures picked out by the term whale (the name for the kind), without knowing the essential features of whales, features likely to involve subtle biological details. Perhaps death, too, is a natural kind whose essence is obscure (a possibility entertained in Chiong 2005, 24–25). A likely challenge to this argument is that we already know a great deal about the physical processes involved in death, making it unlikely that death has a hidden essence the failure to discover which impedes adequate definition.