And in the first place, we remark that the subsistence of finite beings is also contingent and variable. We have before given an idea of subsistence and personality; but we beg leave to recall a few ideas about these most important notions of ideology, that the reader may better perceive in what the nature of the hypostatic union really consists. We shall explain the following notions: possibility, actuality, nature, substance, subsistence, and personality.
Possibility is the non-repugnance of a being. It is intrinsic or exterior. When the essential elements which constitute the idea of a being do not clash together or contradict each other, the being is intrinsically possible. When, besides the intrinsic possibility, there exists a principle which may give the being actual existence, the possibility is external.
The intrinsic possibility of a being in the mind of the cause or principle of this being is called intelligible actuality. Actuality or existence, properly speaking—that is, subjective actuality—is the existence of the being outside of the intelligent cause which perceives it; or, in other words, the external expression of the intelligible actuality.
Nature is the radical, interior principle of action in every existing being.
Substance is the existing of the being in itself, or the permanence and duration of a being in itself. Now, a being which is a substance may be united with another substance, and the union may be so close that one of them may become the natural, inseparable, intrinsic organ of the other. In this case the being which is thus united with the other and has become the organ of the other, although not ceasing to be a substance, possesses no subsistence of its own. What, then, is the subsistence of a being? It is not merely the existing in itself; it is the exclusive possession of the existing in itself and whatever flows from this exclusive possession. A being is possessed of existence in itself and of its operations, when the union of which we have spoken does not exist. But whenever such union exists, though the being continues to be substance or to exist in itself, it has yet no exclusive possession of itself.
Hence, subsistence is defined the last complement of a substance which makes it an independent whole, separate or distinct from all others; makes it own and possess itself, and renders it responsible for its operations. Personality adds to this the element of intelligence; so that a person is that supreme and intelligent principle in a being which knows itself to be a whole, independent of all others; which enjoys the possession of itself, and is responsible for its actions. Consequently, every substance which is complete—that is, detached from and independent of all other substances in such a manner as to constitute a whole by itself, and alone to bear the attribution of its properties, modifications, and functions—is a subsistence.
The subsistence or personality of a contingent being is also contingent, and may be separable from it so as to give rise to a twofold supposition, either that the contingent being never had a subsistence of its own, or, if it had, it may be deprived of it, and its own subsistence may be substituted by another.
In the first place, we remark, in vindication of this statement, that it is impossible that any substance could really exist without a subsistence. Because, as we have said, subsistence is the last complement of substance, and consequently without it the substance could not be actual, but would be a mere abstraction. That for which we contend in the proposition just laid down is, that it is not necessary that a substance should have a subsistence of its own, but that it may subsist of the subsistence of another.
For it is evident that every being comprised within the sphere of the contingent and the finite may cease to be a whole by itself, and may contract with a nature foreign to itself a union so intimate and so strong as to depend on this foreign nature in all its functions and its states, and no longer to bear the attribution and solidarity of its actions and modifications. If, for instance, a hand detached from the whole body were to trace characters, this action would be attributed to it exclusively; it would be a subsistence, a whole by itself, and we should say, That hand writes. But if it should become a part of, and we should consider is as dependent on, a human nature and will, it would then lose the solidary attribution of the function of which it is the organ; and then we could no longer say, That hand writes; but, That man writes.
A contingent substance may be deprived of the possession of its subsistence by a union with a substance even inferior in nature to itself. Because its superiority over this nature would not prevent its being dependent on it in its functions and in its states, as is the case with the human soul, which presides over the body, which produces in it continual changes, and which, in spite of the excellence which distinguishes it from the mass of matter which it animates, yet depends on the body in its most intimate situations, and finds itself bowed down by the continual evil which it suffers thereby.