The second point in the Thomistic doctrine is that corporeal substances are individuated by reason of their _materiality_. The formative, specific, determining principle of the corporeal substance is rendered _incommunicable_ by its union with the material, determinable principle; and it becomes individually _distinct_ or separate by the fact that this latter principle, in order to be capable of union with the given specific form, has in its very essence an exigence for certain more or less determinate dimensions in space. Corporeal things have their natural size within certain limits. The individual of a given corporeal species can exist only because the material principle, receptive of this specific form, has a natural relation to the fundamental property of corporeal things, _viz._ quantity, within certain more or less determinate limits. The form is rendered incommunicable by its reception in the matter. This concrete realization of the form in the matter is individually distinct and separate from other realizations of the same specific form, by the fact that the matter of this realization demands certain dimensions of quantity: this latter property being the root-principle of numerical multiplication of corporeal individuals within the same species.
On the other hand, incorporeal substances such as angels or pure spirits, being “pure” forms, “_formæ subsistentes_,” wholly and essentially unallied with any determinable material principle, are _of themselves_ not only specific but individual; they are themselves essentially incommunicable, superior to all multiplication or repeated realization of themselves: they are such that each can be actualized only “once and for all”: each is a species in itself: it is the full, exhaustive, and adequate expression of a divine type, of an exemplar in the Divine Mind: its realization is not, like that of a material form, the actuation of an indefinitely determinable material principle: it sums up and exhausts the imitable perfection of the specific type in its single individuality, whereas the perfection of the specific type of a corporeal thing cannot be adequately expressed in any single individual realization, but only by repeated realizations; nor indeed can it ever be adequately, exhaustively expressed, by any finite multitude of these.