Abstract
The aim of this article is to help to clarify the role which Aristotle gives to definition in his theory of demonstration. I shall begin by examining his handling of the relations between definition and demonstration in chapters 8-10 of the second book of the Posterior Analytics, in order to provide an outline for an interpretation of Aristotle's thought. Secondly, I shall examine chapter 10 in more detail, bringing out the contrast between the commentary by Averroes and that of Grosseteste. I have chosen these two commentators because, both being generally magnificent interpreters of Aristotle, as far as the nature and types of definition are concerned their understanding of Aristotle is strikingly different.