Two sorts of questions for moderate realism

The vigorous early-fourteenth-century debate about universals was based on a rejection of Platonism, the theory that universal natures really exist independently of the particulars whose natures they are and independently of every mind. Fourteenth-century ‘moderate’ realists agreed that natures must be somehow common to particulars in reality, but Aristotle had convinced them that no one in his right mind could hold that the nature of a thing exists separated from it as Platonic forms were supposed to do. They insisted instead that the natures really exist in the things whose natures they are, as metaphysical constituents of them. But this contention had its own problems. Since there can be more than one particular in a given genus or species, natures cannot be the only metaphysical constituents of particulars; there must also be individuating principles that serve to distinguish one particular from another.

But what are these individuating principles? William of Champeaux's position that accidental properties individuate was denied by virtually everyone on the Aristotelian ground that substance is naturally prior to accidents but particular substances are not naturally prior to what individuates them. Thomas Aquinas held that prime matter, the ultimate property-bearer in composite substances, combines with quantitative dimensions to individuate. But Duns Scotus found this tantamount to conceding that accidents individuate after all. Besides, he argued, neither matter, quantitative dimensions, nor their combination was distinct and determinate in itself. Taking it as axiomatic that only what is distinct and determinate in itself can individuate, Scotus concluded that neither matter by itself, existence, nor any combination of accidents can do the job (Ordinatio II, d. 3, qq. 5, 3, and 4, respectively).