 Substantial forms both unify a substance at a given time and individuate a substance over time. To play this role, they must be particulars rather than  universals.  In general, it seems that Aquinas thinks that no forms are universals except inasmuch as we conceive of them in abstraction from particulars. In external reality (in re), “no commonness is found in Socrates; rather, whatever is in him has been individuated” (On Being and Essence 3.80–2; Leftow 2003; Brower 2016). The principle of individuation, for forms, is matter (King 2000; Klima 2000). The account of individuation, then, comes in two stages: matter individuates the form at the start, when the form first inheres in that matter, and from that point forward the form possesses a fixed identity of its own. If it is a substantial form, it subsequently individuates the substance as a whole.