 Aristotle’s preliminary answer (Ζ.4) to the question “What is substance?” is that substance is essence, but there are important qualifications. For, as he points out, “definition (horismos), like ‘what-it-is’ (ti esti), is said in many ways too” (1030a19). That is, items in all the categories are definable, so items in all the categories have essences—just as there is an essence of man, there is also an essence of white and an essence of musical. But, because of the pros hen equivocity of ‘is’, such essences are secondary—“in the primary (protôs) and unconditional way (haplôs) definition and the essence belong to substances” (1030b4–6). Thus, Ζ.4 tells us, it is only these primary essences that are substances. Aristotle does not here work out the details of this “hierarchy of essences” (Loux, 1991), but it is possible to reconstruct a theory of such a hierarchy on the basis of subsequent developments in Book Ζ.