In Metaphysics Η.6 Aristotle asks: “What unifies a definition, making its parts into a whole and not a mere list?” A definition is a single account, he says, not by being tied together like the Iliad (the whole poem in twenty-four books), but by being of one thing. Although a definition is a complex formula, consisting of a number of linguistic parts, it is one if the object defined is one. So the unity of definition is parasitic on the unity of the object defined. Aristotle is looking for basic entities, whose being is fully determined by their essence and not by other entities which claim priority. These basic entities ground our understanding of other things. What, then, makes an object one, whose account is a definition? Aristotle discusses three sorts of examples: substantial forms, material composites, and categorial properties.

Start with a determinate form such as man. What makes man one? Suppose we define man as “biped animal”: why is man one thing and not an aggregate of two things, animal and biped, which are distinct from each other and logically prior to it – that is, mentioned in its definition? The question threatens the Platonists, who regard animal and biped as separate forms. Aristotle poses the problem thus: