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.