On the Thomistic view of individuation, matter is the individuating factor (or "signate" quantified matter depending on what work of Aquinas you happen to be reading). But this leaves a problem for Thomists in the case of human souls, which are supposed to subsist after death. For there is no matter remaining at all (Aquinas famously rejects spiritual matter). Consequently, we should expect subsistent human souls to be universals, or perhaps to be absorbed in the common nature of humanity (... but Aquinas thinks common natures have no being or unity...). Not so fast says the Thomist (or, if you prefer, the A-T theorist; this subject recently came up on Feser's blog), the soul still has a relation to its body. It is this relation that keeps the soul a particular (other Thomists have told me that the soul is individuated at the instant of its creation and just stays that way). Scotus rejects this line of thought in the passage I have translated below. This question is something of an embarrassment for Scotists of the strict observance (including the editors), for Scotus endorses spiritual matter (that old foolish doctrine that Thomas allegedly refuted for all time). Well, sort of. The following quote is labeled (without evidence) as "ad mentem Guilelmi de la Mare". For details on Scotus' views on spiritual matter (for example, who actually wrote the following section), see our co-blogger Michael's dissertation.
Duns Scotus, Quaestiones de anima, q. 15 n. 10-(Opera Philosophica V, 131-2):
I respond that probably [probabiliter] it can be said that there is matter in the soul, and this according to the Philosopher and those who posit the contrary. One [argument] is that the plurality of individuals in one species requires matter in those individuals, just as is clear from XII Metaphysics, where it is said " that there are not many in the same species moving heaven, because the first does not have matter." This is also clear from diverse [thinkers] positing matter to be the principle of individuation; but in the species of the rational soul there are many individuals, also when it is separated from the body; therefore, etc.
You might say, just as the contrary [party] does, that the soul has matter which it perfects or is made apt to perfect, namely the body. And by reason of the aptitude for diverse perfectible bodies, the [separated soul] can be plurified, not however does it have matter from which it is made.
Against them:
The soul does not exist on account of the body, but rather contrariwise; therefore neither is the distinction nor plurality of souls on account of the distinction of bodies, but rather contrariwise. Whence the Commentator VII Metaphysics says that the members of a lion differ from the members of a deer, because their souls differ; and not contrariwise.
Again, with the foundation or term of a relation destroyed, there is no relation; but that inclination or aptitude to the body is a certain relation; therefore, with the body destroyed after death, there is no inclination of the soul to the body.
The argument is confirmed: because there is no real relation of being to non-being, for relatives are simultaneous in nature; the soul is separated, not however the body which it informs; therefore, etc.
Again, if the distinction of souls is from the side of bodies only, God cannot create two souls without bodies; because there would not be distinguished by bodies, nor also by an inclination to a body; therefore, etc.
Again, every relative form presupposes something absolute in which it is founded; but that inclination to the body is a certain relative form founded in the essence of the soul which is so inclined; therefore the essence of the soul is prior to that inclination; the prior however is not distinguished by the posterior just as neither is it constituted by it, but rather contrariwise; therefore, etc.
Again, that inclination is not of the essence of the soul, because the soul is an absolute nature in itself; therefore it can be understood by an essential understanding [?? intellectu essentiali] without such an inclination, and consequently one is distinguished from another without an inclination to diverse bodies.
Again, because the soul is a 'this', therefore it has such an inclination to this body, not contrariwise; therefore, etc.
