We have reason to think that a fundamental goal of natural science, on Aristotle's view, is to discover the essence-specifying definitions of natural kinds-with biological species as perhaps the most obvious case. However, we have in the end precious little evidence regarding what an Aristotelian definition of the form of a natural kind would look like, and so Aristotle's view remains especially obscure precisely where it seems to be most applicable. I argue that if we can get a better understanding of how the forms of natural kinds are or come to be known, and how they make things intelligible, we can get a better appreciation of the nature of form in general, as well as solve certain puzzles about form and definition.Keywords: Aristotle; definition; hylomorphism; essentialism; natural kinds § 1. The Epistemology of Form and a Puzzle About DefinitionThe resurgence in interest in Aristotelian hylomorphism among both scholars and philosophers has yielded a variety of questions and puzzles about Aristotle's own position, as well as concerns about how hylomorphism in general ought to be developed. The questions about Aristotle's position are not just matters of detail either: they arise from some of his most well-known and foundational commitments regarding form, essence, and definition. He claims, in particular, that forms are the essences of natural beings, 1 that essences are the objects of definition, 2 and that definition is a fundamental goal of scientific knowledge or understanding (epistêmê).3 There is also good reason to think, further, that natural kinds, especially living beings and biological species, represent paradigm cases for Aristotle of substance and definable form. 4 Broadly speaking, then, we have reason to think that a fundamental goal of natural science, on Aristotle's view, is to discover the essence-specifying definitions of natural kinds-with biological species as perhaps the most obvious case. However, we have in the end precious little evidence regarding what an Aristotelian definition of the form of a natural kind would look like, and so Aristotle's view remains especially obscure precisely where it seems to be most applicable.In the various debates to which these questions have given rise, little explicit attention has been paid to epistemological questions about how we are supposed to come to grasp Aristotelian forms. This is surprising, since, for Aristotle, forms are what make nature and natural kinds intelligible, and so an understanding An. 74b5-12, 90a15, 31-34, B 8-10; Phys. 194b16-29, 198a14-35, 200a10-15; Met. 1031b6-7, 1039b31-1040a2. 4 See, e.g. Phys. 192b33-34, 200b3-5; Met. 1028b8-15, 1031a1-14, 1037a10-20, Z 12; Parts of Animals [Part. An.] 639a15-19; cf.Met. 1039a19-20. There are well-known questions and disputes about the relation between forms as they pertain to (biological) species and to the individual members of those species (for example, and perhaps most controversially, whether there are numerically distinct forms of individuals, or only forms of species). F...