This study consists of a series of steps toward the development of a general theory of differences in ontological kind. The first part defines the notion of a "logical individual" and argues for its role as the basic ontological unit. I also take issue with those who hold that 'exist' is equivocal, as well as with those who claim that category-mistakes lack a truth-value. This part concludes with the "existence criterion", according to which a thing exists just in case it (...) is a logical individual; examples of things which satisfy and things which fail to satisfy this criterion are discussed. ;The second part covers a number of topics related to the classification of things into kinds. Problems involved in such classification are considered, including a discussion of whether, and on what grounds, certain attributes may be better qualified to define kinds than are others. The idea of ontological kinds is also adumbrated in an intuitive fashion, and a "formalist" strategy for ontological kinds, under which no attribute is treated as more ontologically puissant than any other, is considered and discarded. ;In the last part the ideal of substance is employed as a way of mediating between individuals, on the one hand, and privileged kinds, on the other. Following an examination of substance in Aristotle, six senses of substance are distinguished, of which two are found to hold particular promise for the development of a theory of ontological kinds. Discussion of these two notions of substance includes critiques of "stuff" metaphysics as well as "Aristotelian essentialism". The latter is found to depend on an unwarranted assumption that certain individuals are not only referentially, pragmatically, or psychologically prior to others, but also ontologically privileged. Refutation of this assumption leads to a rehabilitation of Aristotle's notion of "prime matter", but also to a picture of substance closer to Descartes' than to Aristotle's. The arguments against Aristotelian substance finally suggest an account of ontological kinds in terms of the possibility or impossibility of "fusion"--the coinstantiation by two individuals of a single individual concept. (shrink)