what Thomas called materia signata, i.e. designated matter, matter quantitatively determined, or limited by particular spatial dimensions and boundaries, is, in corporeal substances, the principle of individuation. On the other hand, it is not at all true, as it is sometimes asserted, that, for St. Thomas, matter is the sole principle of individuation in all grades of being. The Thomistic doctrine of the individual, viewed in its wholeness, seems to run much as follows:
An individual (Summa Theol., P. 1, Q. XXX, Art. IV) possesses a certain characteristic modus existendi, in so far as an individual is something “per se subsistens distinctum ab aliis.” Individuals are also to be called, according to the well-known tradition, “first substances” or “hypostases” (Id., Q. XXIX, Art. I). The name “hypostasis,” however, is more properly applied to the rational individual, the person, or to beings “who have dominion over their acts,” or who act per se. The fact of such self-determination gives a peculiar dignity to their individuality; and individuals of this grade are properly called persons, or “hypostases in the proper sense.” Every person is an individual, since actions are “in singularibus” (loc. cit.). On the other hand, not every individual is a person.
If one speaks of the rational individuals, or persons, one observes, then, that their individuality need not be dependent, in any sense, upon material conditions. Thus, according to Thomas (Q. III, Art. 11), a form such as that of God, self-subsistent and not “receivable in matter,” is individuated by the very
