The paper examines the connection between Aristotle's theory of generated substance and his notion of potentiality in Metaphysics Θ.7. Aristotle insists that the matter of a substance is not what that substance is, against a competing view that was widely held both in his day and now. He coined the term thaten (εκεíνινOν) in order to make this point. The term highlights a systematic correspondence between the metaphysics of matter and of quality: the relationship between a thing and its matter is like the relationship between a quali fied thing and the relevant quality. It is argued that Aristotle's view about the matter of particular substances is connected with his view about ultimate matter. His conception of the matter of particular substances allows him to block an argument, from Plato's Timaeus, that ultimate matter must be something imperceptible and lacking all perceptible qualities. Aristotle uses the term thaten to introduce an alternative conception of ultimate matter on which ultimate matter might well be an ordinary perceptible kind of thing.