a) Priority of act to potency. Act and potency are correlatives but there is nevertheless a definite order, a sequence as it were, between them. Act is prior; act accounts for potency, but potency does not account for act as such. In Aristotle's analysis this priority is shown to obtain in several respects.13
First, act is prior in concept or definition; for potency is defined by act, that is, in relation to act, as the ability to build is known from the act of building.
Second, in the order of time a distinction applies. The individual is in potency before being in act; the acorn precedes the oak, the imperfect the perfect. But from the larger point of view of the species the perfect state, which is act, precedes the imperfect state, which is potency. The potentially existent must always come from the actually existent, man from man, etc.
Third, according to substance (or perfection) act again is first, mainly because, in Aristotle's words, "everything that comes to be moves toward its principle and end; for the principle is the final cause, and the becoming is for the sake of the end, but the end is the actuality." 16 In short, the priority of act stems in this case from the priority of final cause, which has obviously to be act.
Lastly, Aristotle interposes an argument that marks a notable advance in his metaphysics as a whole. Eternal beings, he submits (without warning or discussion), are prior to corruptible ones. Then to the point at issue: that eternal beings have no potency to nonbeing, hence are not in potency; which is to say there are beings in act that are prior to all potency. This argument, it may be noted, does more than substantiate the priority of act; it anticipates the pure act, the burden of the subsequent Book Λ.
b) Every activity has its principle in act. Every activity must proceed from a being in act with respect to the aim and scope of the activity.17 This proposition, which is axiomatic in the Aristotelian tradition, follows from the general priority of act spoken of above. The equivalent thought emerges from another Scholastic axiom, that what is in potency cannot be reduced to act unless by a being in act.18
Potency, this means, cannot be raised to the level of act by itself; only a being already in act, exercising efficient causality, can bring this about. Yet, though necessarily in act, the agent must also have the potency to act. Is not this a contradiction? No, it is quite possible to be in act and potency at the same time, but not in the same respect. In a created agent act and potency are complementary. For the exercise of efficient causality the agent must be in act through possession of the form (or perfection) which is to be produced in another; and must at the same time be in (active) potency as regards the action to be performed. So, for instance, the intellect when actuated (or in act) by the impressed species, is in (active) potency as regards the act of intellection.
c) Limitation of act by potency. We have now, following Aristotle, set out the distinction of act and potency, defined the one and the other, noted their principal divisions, and established the priority of act. When we turn to St. Thomas the doctrine of act and potency receives, if anything, even more attention than it does in Aristotle; and certainly, it is put to far greater use. In fact, St. Thomas and not a few Scholastics regard the distinction of act and potency as in some sense, the key principle of metaphysical thought, the answer (or at least, an answer) to the riddle of being. Finite being is explained, basically, as a composition of act and potency; and since infinite being is grasped by us only through the medium of finite being, even theology rests on the doctrine of this distinction.
To say "finite" is, moreover, to say "limited." This brings up the metaphysical question of the limitation of being; or, what is it that limits a being intrinsically? St. Thomas and his followers answer in unison: act can only be limited by potency.19 Thus, in the composition of act and potency act is related to potency as the limited to the limiting.
In proof of this we have the following. Of itself act means perfection. If, then, we find a perfection limited there must be a reason for the limitation. The reason, however, cannot be in the perfection itself; for if perfection is limited by itself then perfection is by nature imperfection, an obvious contradiction. Consequently perfection, if limited, is limited by a principle distinct from yet united with it; this principle is potency. Thus, in every composition of act and potency, act is limited by potency. The corollary to this is that pure act is absolutely unlimited, hence absolutely perfect. This line of reasoning, no doubt, is correct; but St. Thomas, while convinced of the conclusion, comes to it from what would seem to be a more organic and more realistic approach, namely, from the over-all consideration of participated and unparticipated being.
The opposing view held by Scotists and Suarezians should at least be mentioned, that act can be limited in itself. All that is required, then, for the limitation of act is that the extrinsic or efficient cause, ultimately God, should constitute it in this degree of being instead of that. Perhaps, in comment, this view suffers from a lack of attention to the distinction between intrinsic and extrinsic principle of limitation.
d) Multiplication of act by potency. If potency is the (intrinsic) principle of limitation, it is also, and for that very reason, the principle of multiplication. Suppose an act that is not limited by potency; such an act is unlimited or perfect, and therefore unique.20 Unique, because if two beings were equally (and infinitely) perfect, one could not differ from the other; in fact, they would cancel each other. Wherever, then, perfection is plural, the perfection must be limited. But we know it is not limited by itself, hence by something not itself; and this, as we have learned, is potency, which limits act (or perfection) by receiving it and thus makes possible its plurification.
e) Reality of the distinction between act and potency. Scotists and Suarezians maintain that the distinction between act and potency is only "formal," that is, logical, but with a foundation in the things considered (rationis ratiocinatae). For Thomists, however, the distinction is real; and for St. Thomas in particular there could hardly be any question about the matter. The reality of it is generally argued this way. Potency denotes capacity for perfection, whereas act of its very nature signifies perfection; such opposite notions must correspond to entities that are really distinct.
More authentic, perhaps, from the Thomistic stand would be the argument that act, because its reception into potency is by way of causality or participation, must be really distinct from the receiving potency. The distinction, for that matter, is also evidenced experientially, or a posteriori; for sometimes we observe that a potency is without the act it previously had - the sense (potency) of sight, for instance, is not always in the act of seeing. Possibly the Scotist and Suarezian trouble with the real distinction stems from a too literal or too coarse interpretation of it. The composition of act and potency is not a union of two complete beings each of which could exist separately. We have said it before, a real distinction does not necessarily mean that the things in question can exist apart, though what is separable is certainly distinct. Act and potency are not two beings but two principles of being which determine each other but which, while really distinct, do not have each a distinct existence.