 The principal metaphysical questions concerning natural kinds are fourfold. First, are the kinds that we think of as ‘natural’ kinds genuinely natural? Or are our classification processes more anthropocentric than that? Secondly, are our natural kind classifications really classifications into kinds? Thirdly, what are natural kinds? Are natural kinds any sort of entity at all? Are they basic ontological entities or are they derived from or reducible to other entities (e.g., universals)? Fourthly, do natural kinds have essences?