 A distinct approach to artifact kinds is proposed by Thomas Reydon (2014). He points out that the nature of natural kinds is currently in play in philosophy of science. Their mind-independence is traditionally predicated on their having essences. But essentialism ran into trouble when Darwin showed that species—up to that point the very paradigm of natural kinds—are historically fluid and have no clear boundaries. Similar problems have now been recognized even in the kinds of chemistry and physics (Khalidi 2013). Nevertheless, grouping natural objects into kinds does license useful inferences and ground successful explanations. This has led to what Reydon calls an “epistemological turn”.