Aristotle's approach to the definition of composite substances in the central books of the Metaphysics follows the explanation-involving pattern set out in the Posterior Analytics. The definition of such substances draws on resources based in his account of teleological causation The relevant Forms provide the basis of answers to both the questions, ‘What is F?’ and ‘Why is F as it is?’. Neither question can be answered satisfactorily in isolation. Aristotle's explanation-involving account of definition is seen to be at work throughout his discussion in the central books of Metaphysics.
Keywords: actuality, Aristotle, definition, explanation, Form, interdependency, Metaphysics, potentiality, substance