Everything that is something—as Socrates is a man, pale a color, in the garden a location, ten pounds a quantity, or two o’clock a time—is a being. And what makes something a being, according to Aristotle, is its relationship to substance. Hence the account of substance is the centerpiece of his metaphysics, and of his science of being qua being. But the account itself is notoriously difficult—so difficult, in fact, that it is commonly viewed nowadays as an inconsistent amalgam of different accounts, developed at different times.

In the supposedly early Categories, so the story goes, the canonical or primary substances are particular instances (Socrates) of universal species (the human species), which, with genera, are substances only in a secondary sense. In the supposedly later Metaphysics, by contrast, secondary substance is abandoned, Socrates is treated as a compound of matter and form, and form alone is identified as primary substance. Even within the Metaphysics itself, moreover, distinct strata are discernible, with the celebrated middle books (VII–IX)—die Substanzbücher as a recent anthology revealingly calls them—containing Aristotle’s fully mature views. Since divine substance (God) is not discussed in these books, it is not a crucial part of those views, and so may be safely ignored or downplayed. In those same middle books, however, Aristotle claims that primary substances must be both ontologically primary (the fundamental beings) and epistemologically primary (the primary objects of scientific knowledge). At the same time, he is explicit that universals (specifically, definable universal essences) alone enjoy epistemological primacy, since they are the first principles of the sciences. It seems to follow that primary substances must be universals. But this conclusion he just as firmly rejects. The result is a dilemma of sorts, which I call the Primacy Dilemma. In my view, Aristotle’s attempt to solve it is the central project of his entire epistemology and metaphysics—one that leads him to abandon Platonism and become an Aristotelian.