From what has been said it follows that all changeable things, in whatever respect they are subject to change, are compounded of potentiality and act. God alone since he is absolutely unchangeable, is devoid of any potentiality. Since he is subsistent Being itself or the fullness of being, he is incapable of becoming; there is no perfection which he does not possess or rather is not already; he is pure act.
The being of all other things on the contrary, is too poor and too weak to realise simultaneously everything they are capable of being; For every one of them there is really open a vast range of possibilities, of which they can never realise more than a few, and that by changing.
Here we may observe that the obscure and mysterious concept of first matter whose acquaintance we made when we studied the notion of individual nature is that of a pure potentiality in order of substance, which can be any and every body and by itself is none.
It is purely the potential principle which in union with an actual principle (a substantial form), constitutes a particular corporeal substance, and is the subject of substantial changes.
Potentiality and act divide between them the totality of created being , both in the order of substance and in the order of accidents. In other words, they are, like being itself, transcendental objects of thought which exceed or transcend every limitation of class or category, and include all created things. The substance of bodies is compounded of potentiality (first matter) and act (substantial form) . The substance of incorporeal things (pure spirits) , for this substance (substantial essence) is itself potentiality in relation to that which is the ultimate act of everything real (actualitas omnis formae) or existence: pure spirits do not derive their existence from themselves, a se; they cannot be.
On the other hand, we may remark that every accident (whiteness, strength and virtue, etc.) is an act (accidental form) which determines the subject and is itself sometimes in potentiality in respect of further determinations. The intellect, for example, is an accident (accidental form) hose subject is the soul, and it is in potentiality in respect of a particular act of thought.
After what has been said, it is sufficient to consider the notions of potentiality and act to see immediately the truth of the following Axioms:
(3) Act is prior to potentiality. A consequence of the preceding axiom.
(4) Potentiality is essentially relative to act and is for the sake of the act (potentia dictur ad actum). It is indeed only in relation to the act that the potentiality can be conceived (only in relation to being white that we can conceive the power of being white.); and it is also only for the determination or perfection that the determinable and the perfectible as such are.
(5) Act and potentiality belong to the same order., that is to say, both must be in the order of substance, or both in the order of accident. For it is evident that every act which at the same time completes and specifically determines a potentiality must belong to the order of that potentiality.
The activity of thought, for example, belongs to the order of accident like the faculty itself from which it proceeds and which is in potentiality to that activity.
(6) Everything acts according to its nature in act. Since activity is an act (actus operationis) which is brought into being by the subject from which it proceeds it presupposes (as laid down by Axiom ii) that the latter is in act to the extent to which it produces the activity. The same truth differently enunciated is expressed by the dictum action or operation manifests being (operatio sequitur esse).
(7)The combination of two beings in act cannot produce something which is one of itself. We call one of itself (unum per se), as opposed to one by accident, a thing which constitutes a single being and not a conjunction of beings, in other words a thing which is one in virtue of the nature by which it exists. For example , a living organism is a unit of itself, whereas a machine or a house is an accidental unit. This distinction once understood, it is plain that two beings in act, and as such constituting two beings, can never by their combination constitute anything except a conjunction of beings, that is to say , an accidental unit.
No doubt, if you destroy the unity of the machine or the house, you destroy them, but you do not destroy their natures (or substances) of which they are composed (iron, bricks, wood etc.). Destroy, on the other hand, the unity of an organism, you destroy its very nature. (Substance).
As to accidental unit (a conjunction of beings); this axiom plays an important part in natural philosophy and particularly in psychology. For instance, the Cartesian conception which regards the soul and the body as independent of each other, two complete substances, is unable to explain the substantial unity of the human being, because two complete substances are two beings in act.
NB On the question of Act and Potentiality, the school of Aristotle and Thomas teaches the distinction between potentiality and act, the priority of act to potentiality, the reality of motion and becoming, but the priority of Being to motion.
It also shows that between God (pure Act) and all things
else (compounds of potentiality and act) there is an absolute and infinite
difference.