 The principal criteria for being a natural kind used to be metaphysical: a kind is a natural kind if and only if it really exists in the world…independently of human consciousness, human interests, and human practices, and is associated with a particular kind essence…. On the alternative approach the principal criteria for being a natural kind no longer are metaphysical but epistemological: what counts is being useful in human epistemic practices, such as inference and explanation, by corresponding in some way (which is still to be explicated) to the state of affairs in nature. (Reydon 2014, 132)