There are some people who assert that a thing can act only when it is acting, whereas when it is not acting it cannot act, e.g. he who is not building cannot built, but does so the one who is building when he is building; and similarly with all other cases. Now the absurdities that follow this thesis are not hard to be noted.
For it is obvious that one will never be a builder unless he is building, since its being a builder means its being able to build; and similarly with the other arts. If, then, it is impossible for one to possess an art of this sort without having at one time learnt and acquired it, and if it is impossible not to possess it without having lost it once (either by obliviousness or by accident or by time; for certainly this is not happening because of the corruption of the thing itself, since this exists constantly), he will not have got the art when he has ceased acting according to his art. And over again how will he be able to possess it and straightaway start building? The same account holds good for lifeless things; for neither cold nor hot nor sweet nor perceptible at all would ever a thing become unless it had first been perceived. But, indeed, nor even a perception will one be able to obtain if he is not perceiving and acting accordingly. If, then, blind is that which has not got sight, though naturally has, and at the time it is proper for it to have it and insofar as it still exists, then the same people would become many times in the day blind, and deaf too.
Moreover, if that which is deprived of potency is incapable, that which is not coming to be will be incapable of coming to be, and saying that what is incapable of coming to be either that it is or will be is false; for this was what incapable meant. So these views negate both movement and coming to be. For what stands will always stand, and what sits will always sit; for it will not get up insofar as it is sitting, since it will be impossible for what is incapable of getting up to get up. If, then, these views cannot stand, it is clear that potency and actuality are different things; instead, they present potency and actuality as being the same, so what they are seeking to negate is not a small thing.
Therefore, a thing may be capable of not being, though capable of being, and of being, though capable of not being; and regarding the other categories likewise while capable of walking, yet not walk, and while capable of not walking, yet walk. And what is capable of doing something is that, in which if the actuality of that of which it is said to have the potency is present, nothing will be impossible. I mean, for example, if something is capable of sitting and may sit, if the above mentioned actuality is present in it, nothing will be impossible; and similarly if it is capable of being moved, or of moving, or of standing, or of making to stand, or of being, or of coming to be, or of not being, or of not coming to be.
The name “actuality”, which is conceived together with the actualization of the end, is mostly derived from movements, and therefrom to the others; for actuality in its common sense is mostly referred to movement. This is why to things that have no existence we do not ascribe movement as a predicate, but we do ascribe other predicates, e.g. we call them intelligible and desirable, yet not movable. This is happening because they would be understood as being in actuality without them being so. For of things that have no existence some exist potentially; but they do not exist, because they are not in complete reality.
Bibliography: Aristotle, Metaphysics (1046b.29 to 1047b.2)
Translation: George Kotsalis