 Here I assume, with Scotus, that a haecceity is a constituent of the item it individuates. I will return later on to theories that make the individuation of particulars primitive. Scotus believes that the haecceity must be a constituent, and the reason is that he grants some kind of reality to universals too (common natures, in the technical medieval parlance) – and that the reality of a common nature requires the reality of the haecceity, and that both be constituents of the particular. So, haecceities are bare particulars, but they are not supposed to underlie properties. Some medieval Aristotelians hold that matter, construed as a substrate for forms and properties, individuates; and it would be possible to hold that the individuation of a substance is explained by some bare particular that underlies properties. Scotus rejects the view that matter could be responsible for individuation, on the grounds that we require too an explanation for the individuation of (chunks of) matter (Scotus, Ordinatio II, d. 3, p. 1, qq. 5–6, n. 187 (Scotus [OO], 7:483; Spade (1994), 106–107)). Neither would Scotus have been able to make any sense of a bare particular in the sense of a bare possessor of qualities, a view that would doubtless have struck him as inconsistent with essentialism (on this, see Park [1990]). Given that the argument for the reality of haecceities (as constituents of particulars) rests on the reality of common natures, we have first to provide an explanation of Scotus’s theory of common natures, followed, in a third section, by a description of the theory of haecceity and the relation of these two theories to each other.