God alone is pure act: real being divides into actual, potential, and mixed

Article III. The third division of real being is set out, and it is explained how God is pure act.

1. Third, real being is divided into that which is merely actual, that which is merely potential, and that which is partly actual, partly potential — or mixed from potency and act. Merely potential being is that which does not exist in act, but can exist: such a being is a rose in winter. It is to be noted here that philosophers commonly say that those beings which do not exist in act, but can really exist, are in objective potency. For the object of the productive potency which is in causes is that which is producible by the causes — or that which has not yet received existence from the causes, but can receive it. Therefore some possible being, so long as it does not exist in act, is the object of the productive potency of causes, and for that reason is said to be in objective potency, because namely it has that potency which belongs to things so long as they are objects of some causative potency. But when that being has been produced in act and has received existence from causes, it is no longer the object of the productive potency of causes, and on that account is said not to be in objective potency.

2. Merely actual being, or pure act, is the glorious God, to whom this supreme and perfect actuality is ascribed: first, to mark his simplicity. For every composition from potency and act is repugnant to him — that is, from some potential or imperfect principle, and an actual principle; that is, one that actuates and perfects that other principle, as we shall later declare in the doctrine concerning God. Second, to remove and exclude every potency that argues some imperfection. Now there are three kinds of potencies which, on account of their imperfection, cannot be ascribed to God: namely objective potency, which is nothing other than the producibility of a thing by its causes; second, passive potency, which belongs to things insofar as they can suffer and be changed by some agent; third, potency toward non-being. The first potency belongs to things that do not exist, and therefore cannot be ascribed to God, who exists necessarily and never could fail to exist; the two latter belong to things that do exist, yet are mutable and passible: but God is immutable and impassible. But of active potency the account is far otherwise. For active potency, as the metaphysicians have well observed, rather deserves the name of virtue or faculty than of potency: for each thing acts insofar as it is in act, whence the very power of acting is a certain actuality. But someone will ask: every potency is more imperfect than its act; and whatever is imperfect, whether absolutely or comparatively, is to be removed from God. It is answered that even active potency in creatures is often more imperfect than its act, and that because the perfection of a creature is placed in the operation and exercise of the potency, and it is more perfect when it acts than when it does not act: but God does not stand thus. For his supreme and infinite perfection, or beatitude, is placed in this — that he is God — and therefore he does not stand better or more perfectly when he acts than when he does not act.

3. Being that consists of potency and act is finite being actually existing in the world. And a being of this sort is said to consist of act and potency, or to be partly actual, partly potential, for two reasons. First, because in finite beings various compositions are found: namely from genus and difference, from matter and form, from integral parts, from essence and existence; from existing essence and subsistence superadded to it, from subject and accident. Now in all these compositions (that alone excepted which is from integrating parts) there is one potential and receptive principle, which on that account is called potency, and another actual and perfective principle, which on that account is called act: the principle of the former kind underlies in every composition, and the principle of the latter kind is superadded to it. Hence it is plain that finite beings are rightly said to be composed of potency and act. The second reason is this: finite beings, although they exist in act and thus have some act and perfection, are nevertheless not merely actual and perfect, but have those three kinds of potencies that argue imperfection. For first, before they exist, they have that objective potency toward existing. Second, when they exist, they have passive potency, and a receptive potency for new accidents. Third, they have potency toward non-being: for they can be abolished by the first cause and reduced to nothing.