 At this point, we seem to have a clear idea about the nature of substantial form as Aristotle conceives of it. A substantial form is the essence of a substance, and it corresponds to a species. Since it is an essence, a substantial form is what is denoted by the definiens of a definition. Since only universals are definable, substantial forms are universals. That substantial forms are universals is confirmed by Aristotle’s comment, at the end of Ζ.8, that “Socrates and Callias … are distinct because of their matter … but the same in form” (1034a6–8). For them to be the same in form is for them to have the same form, i.e., for one and the same substantial form to be predicated of two different clumps of matter. And being “predicated of many” is what makes something a universal (De Interpretatione 17a37).