We must farther explain in what cases it is proper to say that a thing is δυνάμει, and in what cases it is not proper. You cannot properly say that earth is potentially a man: you may perhaps say that the semen is potentially a man; yet even this not certainly, since other conditions besides semen are required (p. 1049, a. 2). The physician cannot cure every patient, yet neither is the cure altogether a matter of chance (ἀπὸ τύχης — a. 4): there is a certain measure of cure possible, and that is called τὸ ὑγιαῖνον δυνάμει. The definition thereof, taken from the side of the agent, would be — that which will come to pass if he wills it, without any impediment from without; from the side of the patient — when no impediment occurs from within him (a. 8). In like manner, a house exists δυνάμει, when all the matter for it is brought together, without need either of addition or subtraction or change, and when there is no internal impediment; and so with other products of art, where the principle of generation is extrinsic to themselves. In natural products, where the principle of generation is intrinsic, we treat them as potentially existing, when this principle is in a condition to realize itself through itself, assuming no external impediments to interfere. Thus we do not call the semen potentially a man, because, before it becomes such, it must undergo change in something else, and therefore stands in need of some other principle; we call it so only when it is in such conditions that its own principle suffices. Earth is not said to be a statue δυνάμει, until it has first been changed into brass (a. 17). We call the product not by the name of the Matter itself, but by an adjective appellation derived from the next adjacent Matter; thus we call a box, not wood, but wooden: wood is then a box δυνάμει. But we say this only of the proximate or immediate Matter, not of the remote or primary Matter. We must go back through successive stages to the first or most remote Matter; thus wood is not earth, but earthy: earth therefore is potentially wood. The earth may be aeriform; the air may be fiery; the fire has no analogous adjective whereby it can be called, and is thus the first or last Matter. But it is not said to be potentially any thing except the σύνθετον combined with Form immediately above it. Matter may be either proximate or remote: Potentiality is affirmed only of the proximate Matter.
Since all the different meanings of Prius have been enumerated and distinguished, it is plain that in all those meanings Actuality is prius as compared with Potentiality: whether the δύναμις be ἀρχὴ μεταβλητικὴ ( = κινητικὴ) ἐν ἄλλῳ ᾗ ἄλλο, like Art; or ἀρχὴ κινητικὴ ἢ στατικὴ ἐν αὐτῷ ᾗ αὐτό, like Nature (p. 1049, b. 5-10). Actuality is prius both λόγῳ and οὐσίᾳ: it is also prius χρόνῳ in a certain sense, though not in a certain other sense.
It is prius λόγῳ, because the Actual is included in the definition of the Potential; that is, it must be presupposed and foreknown, before you can understand what the Potential is (p. 1049, b. 17). You explain οἰκοδομικός or ὁρατικός by saying that he is δυνάμενος οἰκοδομεῖν ἢ ὁρᾶν: you explain ὁρατόν by saying that it is δυνατὸν ὁρᾶσθαι: τὸ δυνατόν, in its first and absolute meaning, is δυνατόν because it may come into Actuality (b. 13).
It is prius χρονῷ in the sense that the Potential always presupposes an Actual identical specie, though not identical numero, with that Actual to which the Potential tends. Take a man now existing and now seeing, or corn now ripe in the field: these doubtless, before they came into their present condition, must have pre-existed in Potentiality; that is, there must have pre-existed a certain matter — seed or a something capable of vision — which at one time was not yet in a state of Actuality (p. 1049, b. 23). But prior to this matter there must have existed other Actualities, by which this matter was generated: the Actual is always generated out of its Potential by a prior Actual, e.g., a man by a man, a musical man by a musical man; there being always some prior movent, which must be itself already in Actuality (b. 27). We have already declared that every thing generated is something generated out of something, and by something which is identical in species with the thing generated (b. 29). Hence it seems that there can be no builder who has built nothing, no harper who has never harped; for the man who is learning to harp learns by harping (b. 32); which gave occasion to the sophistical puzzle — That one, who does not possess the knowledge, will nevertheless do that to which the knowledge relates. The learner does not possess the knowledge; yet still he must have possessed some fragments of the knowledge: just as, in every thing which is in course of generation, some fraction must have been already generated; in every thing which is moved, some fraction has been already moved (b. 36).
Lastly, Actuality is prius as compared with Potentiality (not merely λόγῳ, καὶ χρόνῳ ἔστιν ὥς, but also) οὐσίᾳ (p. 1050, a. 4). In the first place, that which is latest in generation is first in Form and in Essence; a man compared with a child, man as compared with semen. Man already possesses the Form, semen does not. Next, every thing generated marches or gradually progresses towards its principle and towards its end. The principle is the οὗ ἕνεκα, and the generation is for the sake of the end. Now the end or consummation is Actuality, and for the sake of this the Potentiality is taken on (λαμβάνεται — a. 10). Animals do not see in order that they may have sight; they have sight in order that they may see: they do not theorize in order that they may possess theoretical aptitude, but the converse; except indeed those who are practising as learners. Moreover, Matter is said to exist potentially, because it may come into Form; but, when it exists actually, it is then in Form (a. 16). (Alexander says: ὥστε κἂν τούτῳ προτέρα (ἡ ἐνέργεια) ὡς ἐφετὸν καὶ τάσσον καὶ εἰς κόσμον ἄγον δυνάμεως — p. 559, 10, Bon.) The case is the same where the end is nothing beyond a particular mode of motion (e.g., dancing): the dancing-master has attained his end when he exhibits his pupil actually dancing. In natural productions this is no less true than in artificial: Nature has attained her end, when the product comes into ἐνέργεια; that is, when it is actually at work, from whence the name ἐνέργεια is derived (τὸ γὰρ ἔργον τέλος, ἡ δὲ ἐνέργεια τὸ ἔργον — καὶ συντείνει πρὸς τὴν ἐντελέχειαν — a. 23).
In some cases (as we have often remarked) the ultimatum is use, without any ulterior product distinct from the use, e.g., the act of seeing is the ultimatum of the visual power (p. 1050, a. 24); in other cases there is something ulterior and distinct as a house from the building power. In the former of these cases, Actuality is the end of δύναμις; in the latter it is more the end than δύναμις. (Ὅμως οὐθὲν ἧττον ἔνθα μὲν τέλος ἔνθα δὲ μᾶλλον τέλος τῆς δυνάμεώς ἐστιν· ἡ γὰρ οἰκοδόμησις ἐν τῷ οἰκοδομουμένῳ, καὶ ἅμα γίγνεται καὶ ἔστι τῇ οἰκίᾳ — a. 29. This passage is obscure: see the comments of Alexander, with the notes of Schwegler and Bonitz, who accuse Alexander of misunderstanding it; though it appears to me that neither of them is quite clear. I understand Aristotle to reason as follows:— Ὅρασις is the τέλος, the ἐνέργεια, the consummation of the visual power called ὄψις; but οἰκοδόμησις, is not the τέλος, the ἐνέργεια, the consummation of the building power called οἰκοδομική. This last has its τέλος, ἐνέργεια, consummation, in the ulterior product οἰκία. Nevertheless οἰκοδόμησις, residing as it does ἐν τῷ οἰκοδομουμένῳ, and coming into existence simultaneously with the house, is more the end, more akin to the end or consummation than the building power called οἰκοδομική.)
In cases where there is an ulterior product beyond and apart from the exercise of the power, the Actuality (consummation) resides in that product (p. 1050, a. 31). In cases where is no such ulterior product, the Actuality resides in the same subject wherein the power resides. Thus sight resides in him who sees, and life in the soul. Hence also happiness resides in the soul; for happiness is a certain kind of life (b. 1).
It is thus plain that Actuality is the Essence and the Form, and that it is prius τῇ οὐσίᾳ compared with Potentiality. And, as has been already remarked, one Actuality always precedes another, in time, up to the eternal Prime Movent (p. 1050, b. 5). Moreover, ἐνέργεια is prius to δύναμις in respect to speciality and dignity (κυριωτέρως — b. 6). For eternal things are priora in essence to destructible things, and nothing is eternal δυνάμει, as the reason of the case will show us (b. 8).
All Potentiality applies at once to both sides of the Antiphasis — to the affirmative as well as to the negative. That which is not possible, will never occur to any thing; but every thing which is possible may never come to Actuality (τὸ δυνατὸν δὲ πᾶν ἐνδέχεται μὴ ἐνεργεῖν — p. 1050, b. 10). That which is possible to be, is also possible not to be. Now that which is possible not to be, may perhaps not be (ἐνδέχεται μὴ εἶναι — b. 13); but that which may not be, is destructible, either absolutely (that is, in respect to Essence), or in respect to such portions of its nature as may not be, that is, in respect to locality or quantity or quality. Accordingly, of those things which are absolutely, or in respect to Essence, indestructible, nothing exists δυνάμει absolutely or in respect to Essence, though it may exist δυνάμει in certain respects, as in respect to quality or locality); all of them exist ἐνεργείᾳ (b. 18). Nor does any thing exist δυνάμει, which exists by necessity; yet the things which exist by necessity are first of all (i.e., priora in regard to every thing else); for, if they did not exist, nothing would have existed. Moreover, if there be any Eternal Motion, or any Eternal Motum, it cannot be Motum δυνάμει except in respect to whence and whither; in that special respect, it may have Matter or Potentiality (b. 21).
Accordingly, the Sun, the Stars, and the whole Heaven, are always at work, and there is no danger of their ever standing still, which some physical philosophers fear (ἀεὶ ἐνεργεῖ ὁ ἥλιος — p. 1050, b. 22); nor are they fatigued in doing this. Motion with them is not a potentiality of both members of the Antiphasis, either to be moved or not to be moved. If the fact were so — if their Essence were Matter and Power, and not Act — the perpetual continuity of (one side of the alternative) motion would be toilsome to them; but it is not toilsome, since Actuality is their very Essence (b. 28). Likewise mutable things (which are destructible), such as earth and fire, imitate these indestructible entities, being ever at work; for these elements possess motion by themselves and in themselves, each changing into another (b. 30; compare De Gen. et Corr. p. 337, a. 2). But the other δυνάμεις are all potentialities of both sides of the Antiphasis, or of both alternatives. The rational δυνάμεις can cause motion in such and such way, or not in such and such way; the irrational δυνάμεις may be present or absent, and thus embrace both sides of the alternative (b. 33).