Now since some things can exist in separation and others cannot, it is the former that are substances. 1071aAnd therefore all things have the same causes, because without substance there can be no affections and motions. Next we shall see¹ that these causes are probably soul and body, or mind, appetite and body.² Again, there is another sense in which by analogy the principles are the same viz. actuality and potentiality; but these are different for different things, and apply to them in different ways. For in some cases the same thing exists now actually and now potentially; e.g. wine or flesh or man (actuality and potentiality also fall under the causes as already described; for the form exists actually if it is separable, and so does the compound of form and matter, and the privation, e.g. darkness or disease; and the matter exists potentially, for it is this which has the potentiality of becoming both³; but the distinction in virtue of actuality and potentiality applies in a different sense to cases where the matter of cause and effect is not the same, in some of which the form is not the same but different. E.g., the cause of a man is (i) his elements: fire and earth as matter, and the particular form; (2) some external formal cause, viz. his father; and besides these (3) the sun and the ecliptic,⁴ which are neither matter nor form nor privation nor identical in form with him, but cause motion. Further, we must observe that some causes can be stated universally, but others cannot. The proximate principles of all things are the proximate actual individual and another individual which exists potentially.⁵ Therefore the proximate principles are not universal. For it is the particular that is the principle of particulars; “man” in general is the principle of “man” in general, but there is no such person as “man,” whereas Peleus is the principle of Achilles and your father of you, and this particular B of this particular BA; but B in general is the principle of BA regarded absolutely. Again, even if the causes of substances are universal, still, as has been said,⁶ different things, i.e. things which are not in the same genus, as colors, sounds, substances and quantity, have different causes and elements, except in an analogical sense; and the causes of things which are in the same species are different, not in species, but because the causes of individuals are different: your matter and form and moving cause being different from mine, although in their universal formula they are the same. As for the question what are the principles or elements of substances and relations and qualities, whether they are the same or different, it is evident that when the terms “principle” and “element” are used with several meanings they are the same for everything; but when the meanings are distinguished, they are not the same but different; except that in a certain sense they are the same for all. In a certain sense they are the same or analogous, because (a) everything has matter, form, privation and a moving cause; (b) the causes of substances may be regarded as the causes of all things, since if substances are destroyed everything is destroyed; and further (c) that which is first in complete reality⁷ is the cause of all things. In another sense, however, proximate causes are different; there are as many proximate causes as there are contraries which are predicated neither as genera nor with a variety of meanings⁸; and further the particular material causes are different. 1071bThus we have stated what the principles of sensible things are, and how many they are, and in what sense they are the same and in what sense different.

Since we have seen⁹ that there are three kinds of substance, two of which are natural and one immutable, we must now discuss the last named and show that there must be some substance which is eternal and immutable. Substances are the primary reality, and if they are all perishable, everything is perishable. But motion cannot be either generated or destroyed, for it always existed¹⁰; nor can time, because there can be no priority or posteriority if there is no time.¹¹ Hence as time is continuous, so too is motion; for time is either identical with motion or an affection of it.¹² But there is no continuous motion except that which is spatial, of spatial motion only that which is circular.¹³ But even if we are to suppose that there is something which is kinetic and productive although it does not actually move or produce, there will not necessarily be motion; for that which has a potentiality may not actualize it. Thus it will not help matters if we posit eternal substances, as do the exponents of the Forms, unless there is in them some principle which can cause change.¹⁴ And even this is not enough, nor is it enough if there is another substance besides the Forms; for unless it actually functions there will not be motion. And it will still not be enough even if it does function, if its essence is potentiality; for there will not be eternal motion, since that which exists potentially may not exist. Therefore there must be a principle of this kind whose essence is actuality. Furthermore these substances¹⁵ must be immaterial; for they must be eternal if anything is. Therefore they are actuality. There is a difficulty, however; for it seems that everything which actually functions has a potentiality, whereas not everything which has a potentiality actually functions; so that potentiality is prior. But if this is so, there need be no reality; for everything may be capable of existing, but not yet existent. Yet if we accept the statements of the cosmologists who generate everything from Night,¹⁶ or the doctrine of the physicists that “all things were together,”¹⁷ we have the same impossibility; for how can there be motion if there is no actual cause? Wood will not move itself—carpentry must act upon it; nor will the menses or the earth move themselves—the seeds must act upon the earth, and the semen on the menses. Hence some, e.g. Leucippus¹⁸ and Plato,¹⁹ posit an eternal actuality, for they say that there is always motion; but why there is, and what it is, they do not say; nor, if it moves in this or that particular way, what the cause is. For nothing is moved at haphazard, but in every case there must be some reason present; as in point of fact things are moved in one way by nature, and in another by force or mind or some other agent. And further, what kind of motion is primary? For this is an extremely important point. 1072aAgain, Plato at least cannot even explain what it is that he sometimes thinks to be the source of motion, i.e., that which moves itself; for according to him the soul is posterior to motion and coeval with the sensible universe.²⁰ Now to suppose that potentiality is prior to actuality is in one sense right and in another wrong; we have explained²¹ the distinction. But that actuality is prior is testified by Anaxagoras (since mind is actuality), and by Empedocles with his theory of Love and Strife, and by those who hold that motion is eternal, e.g. Leucippus. Therefore Chaos or Night did not endure for an unlimited time, but the same things have always existed, either passing through a cycle or in accordance with some other principle—that is, if actuality is prior to potentiality. Now if there is a regular cycle, there must be something²² which remains always active in the same way; but if there is to be generation and destruction, there must be something else²³ which is always active in two different ways. Therefore this must be active in one way independently, and in the other in virtue of something else, i.e. either of some third active principle or of the first. It must, then, be in virtue of the first; for this is in turn the cause both of the third and of the second. Therefore the first is preferable, since it was the cause of perpetual regular motion, and something else was the cause of variety; and obviously both together make up the cause of perpetual variety. Now this is just what actually characterizes motions; therefore why need we seek any further principles?

Since (a) this is a possible explanation, and (b) if it is not true, we shall have to regard everything as coming from “Night”²⁴ and “all things together” and “not-being,”²⁵ these difficulties may be considered to be solved. There is something which is eternally moved with an unceasing motion, and that circular motion. This is evident not merely in theory, but in fact. Therefore the “ultimate heaven” must be eternal. Then there is also something which moves it. And since that which is moved while it moves is intermediate, there is something which moves without being moved; something eternal which is both substance and actuality. Now it moves in the following manner. The object of desire and the object of thought move without being moved. The primary objects of desire and thought are the same. For it is the apparent good that is the object of appetite, and the real good that is the object of the rational will.²⁶ Desire is the result of opinion rather than opinion that of desire; it is the act of thinking that is the starting-point. Now thought is moved by the intelligible, and one of the series of contraries²⁷ is essentially intelligible. In this series substance stands first, and of substance that which is simple and exists actually. (The one and the simple are not the same; for one signifies a measure,²⁸ whereas “simple” means that the subject itself is in a certain state.) But the Good, and that which is in itself desirable, are also in the same series; 1072band that which is first in a class is always best or analogous to the best. That the final cause may apply to immovable things is shown by the distinction of its meanings. For the final cause is not only “the good for something,” but also “the good which is the end of some action.” In the latter sense it applies to immovable things, although in the former it does not; and it causes motion as being an object of love, whereas all other things cause motion because they are themselves in motion. Now if a thing is moved, it can be otherwise than it is. Therefore if the actuality of “the heaven” is primary locomotion, then in so far as “the heaven” is moved, in this respect at least it is possible for it to be otherwise; i.e. in respect of place, even if not of substantiality. But since there is something—X—which moves while being itself unmoved, existing actually, X cannot be otherwise in any respect. For the primary kind of change is locomotion,²⁹ and of locomotion circular locomotion³⁰; and this is the motion which X induces. Thus X is necessarily existent; and qua necessary it is good, and is in this sense a first principle.³¹ For the necessary has all these meanings: that which is by constraint because it is contrary to impulse; and that without which excellence is impossible; and that which cannot be otherwise, but is absolutely necessary.³² Such, then, is the first principle upon which depend the sensible universe and the world of nature. And its life is like the best which we temporarily enjoy. It must be in that state always (which for us is impossible), since its actuality is also pleasure.³³(And for this reason waking, sensation and thinking are most pleasant, and hopes and memories are pleasant because of them.) Now thinking in itself is concerned with that which is in itself best, and thinking in the highest sense with that which is in the highest sense best.³⁴ And thought thinks itself through participation in the object of thought; for it becomes an object of thought by the act of apprehension and thinking, so that thought and the object of thought are the same, because that which is receptive of the object of thought, i.e. essence, is thought. And it actually functions when it possesses this object.³⁵ Hence it is actuality rather than potentiality that is held to be the divine possession of rational thought, and its active contemplation is that which is most pleasant and best. If, then, the happiness which God always enjoys is as great as that which we enjoy sometimes, it is marvellous; and if it is greater, this is still more marvellous. Nevertheless it is so. Moreover, life belongs to God. For the actuality of thought is life, and God is that actuality; and the essential actuality of God is life most good and eternal. We hold, then, that God is a living being, eternal, most good; and therefore life and a continuous eternal existence belong to God; for that is what God is. Those who suppose, as do the Pythagoreans and Speusippus,³⁶ that perfect beauty and goodness do not exist in the beginning (on the ground that whereas the first beginnings of plants and animals are causes, it is in the products of these that beauty and perfection are found) are mistaken in their views. For seed comes from prior creatures which are perfect, and that which is first is not the seed but the perfect creature. 1073aE.g., one might say that prior to the seed is the man—not he who is produced from the seed, but another man from whom the seed comes.³⁷ Thus it is evident from the foregoing account that there is some substance which is eternal and immovable and separate from sensible things; and it has also been shown that this substance can have no magnitude, but is impartible and indivisible (for it causes motion for infinite time, and nothing finite has an infinite potentiality³⁸; and therefore since every magnitude is either finite or infinite, it cannot have finite magnitude, and it cannot have infinite magnitude because there is no such thing at all); and moreover that it is impassive and unalterable; for all the other kinds of motion are posterior to spatial motion. Thus it is clear why this substance has these attributes.