 Ζ.3 begins with a list of four possible candidates for being the substance of something: essence, universal, genus, and subject. Presumably, this means that if x is a substance, then the substance of x might be either (i) the essence of x, or (ii) some universal predicated of x, or (iii) a genus that x belongs to, or (iv) a subject of which x is predicated. The first three candidates are taken up in later chapters, and Ζ.3 is devoted to an examination of the fourth candidate: the idea that the substance of something is a subject of which it is predicated.