In this paper, I attempt to set out some of the main views about essence which Aristotle puts forward in Book Z of theMetaphysics.A central feature of Aristotle's account, as we shall see, is his distinction between primary and secondary cases of essence, and a similiar distinction in connection with the related notions of substance and definition. This division between primary and secondary cases means that Aristotle may well have two very different sorts of remark to make about essence, or (...) definition, or substance, according to which of the two kinds of case he has in mind. There is no reason to suppose the same logical properties apply to both the primary and the secondary cases. And when we come to define the various notions, we shall see that if our definitions are right, the logical properties in the two kinds of case are in fact quite different. (shrink)