  As we have seen, Aristotle insists upon the primacy of primary substance in his Categories. According to that work, however, star instances of primary substance are familiar living beings like Socrates or an individual horse (Cat. 2a11014). Yet with the advent of hylomorphism, these primary substances are revealed to be metaphysical complexes: Socrates is a compound of matter and form. So, now we have not one but three potential candidates for primary substance: form, matter, and the compound of matter and form. The question thus arises: which among them is the primary substance?  Is it the matter, the form, or the compound?  The compound corresponds to a basic object of experience and seems to be a basic subject of predication: we say that Socrates lives in Athens, not that his matter lives in Athens. Still, matter underlies the compound and in this way seems a more basic subject than the compound, at least in the sense that it can exist before and after it does. On the other hand, the matter is nothing definite at all until enformed; so, perhaps form, as determining what the compound is, has the best claim on substantiality. 