This thesis investigates Aristotle's criticism and consequent reformulation of the Platonic method for formulating definitions called the Method of Division. For both Plato and Aristotle, the object of division is a natural kind, which consists in a class whose members stand in a homologous relationship to a single form. I argue that Aristotle's criticisms of the Method of Division fall under two categories: logical objections and ontological objections. The logical objections focus on division as a method for demonstrating definitions, a method that Aristotle wants to distinguish from his syllogistic logic, the centerpiece of his theory of scientific demonstration. The ontological objections focus on the question of whether the sort of account generated by division is sufficient to constitute a definition of its object. Aristotle's revised Method of Division is supposed to avoid the problems he raises by constructing definitions that satisfy the principles motivating his ontological objections through a logical process devised to make the resulting account a "necessary" consequence of the initial assumptions of the division. I argue that Aristotle?s ontological objections to the Method of Division reflect a deeper disparity between the Platonic and the Aristotelian notion of a form and natural kind. Underpinning Aristotle's notion of a natural kind is an ontology of discrete substances. Because the unity of substance is paramount in this ontology, Aristotle argues that a definition, which is supposed to give an account of the essence of a substance, must account for the unity of its object by itself possessing a non-accidental unity. Yet, on a Platonic ontology, a definition by division invokes a plurality of independent Forms whose conjunction does not constitute a unity. On the basis of this consideration, Aristotle argues that an ontology of abstract Forms cannot account for the unity of an individual substance. To this extent, I conclude, Aristotle's methodological objections to the Platonic Method of Division are a component of his broader criticisms of Platonic metaphysics.