 As regards the first question, the general problem is to determine which of the kinds to which science makes appeal, if any, correspond to real natural kinds—those existing in nature, so to speak—and which of these kinds are merely conventional—those whose boundaries are fixed by us rather than nature. The second question builds on the first by noting that in addition to classifying things in a natural way, classification by natural kinds, is also classification of things into kinds. What then is it to classify by kind? Then, if natural kinds are genuinely natural and genuinely kinds, what are they? Are there entities that are the natural kinds, and if so what can we say about the nature of such entities? Are they irreducible, basic, sui generis entities (alongside, for example, particulars and universals) (Ellis 2001; Lowe 1998)? Or can they be reduced, say to universals (Armstrong 1978, 1997; Hawley and Bird 2011) or to clusters of properties (Boyd 1991, Millikan 1999)? The third question asks whether there are properties that might be essential for kind membership. Natural kind essentialists hold that natural kinds have essences (Ellis 2001, 2002; Devitt 2008), where the essence of a natural kind is a property or set of properties whose possession is a necessary and sufficient condition for a particular’s being a member of the kind. That fact is a so-called essential fact concerning the kind; it is a fact that, in Fine’s terms, stems from the identity or nature of the kind (Fine 1994).