We must, however, distinguish when a particular thing exists potentially, and when it does not; for it does not so exist at any and every time. 1049aE.g., is earth potentially a man? No, but rather when it has already become semen,¹ and perhaps not even then; just as not everything can be healed by medicine, or even by chance, but there is some definite kind of thing which is capable of it, and this is that which is potentially healthy. The definition of that which as a result of thought comes, from existing potentially, to exist actually, is that, when it has been willed, if no external influence hinders it, it comes to pass; and the condition in the case of the patient, i.e. in the person who is being healed, is that nothing in him should hinder the process. Similarly a house exists potentially if there is nothing in X, the matter, to prevent it from becoming a house, i.e., if there is nothing which must be added or removed or changed; then X is potentially a house; and similarly in all other cases where the generative principle is external. And in all cases where the generative principle is contained in the thing itself, one thing is potentially another when, if nothing external hinders, it will of itself become the other. E.g., the semen is not yet potentially a man; for it must further undergo a change in some other medium.² But when, by its own generative principle, it has already come to have the necessary attributes, in this state it is now potentially a man, whereas in the former state it has need of another principle; just as earth is not yet potentially a statue, because it must undergo a change before it becomes bronze. It seems that what we are describing is not a particular thing, but a definite material; e.g., a box is not wood, but wooden material,³ and wood is not earth, but earthen material; and earth also is an illustration of our point if it is similarly not some other thing, but a definite material—it is always the latter term in this series which is, in the fullest sense, potentially something else. E.g., a box is not earth, nor earthen, but wooden; for it is this that is potentially a box, and this is the matter of the box—that is, wooden material in general is the matter of “box” in general, whereas the matter of a particular box is a particular piece of wood. If there is some primary stuff, which is not further called the material of some other thing, this is primary matter. E.g., if earth is “made of air,” and air is not fire, but “made of fire,” then fire is primary matter, not being an individual thing. For the subject or substrate is distinguishable into two kinds by either being or not being an individual thing. Take for example as the subject of the attributes “man,” or “body” or “soul,” and as an attribute “cultured” or “white.” Now the subject, when culture is induced in it, is called not “culture” but “cultured,” and the man is called not whiteness but white; nor is he called “ambulation” or “motion,” but “walking” or “moving”; just as we said that things are of a definite material. Thus where “subject” has this sense, the ultimate substrate is substance; but where it has not this sense, and the predicate is a form or individuality, the ultimate substrate is matter or material substance. It is quite proper that both matter and attributes should be described by a derivative predicate, 1049bsince they are both indefinite. Thus it has now been stated when a thing should be said to exist potentially, and when it should not.

Now since we have distinguished⁴ the several senses of priority, it is obvious that actuality is prior to potentiality. By potentiality I mean not that which we have defined as “a principle of change which is in something other than the thing changed, or in that same thing qua other,” but in general any principle of motion or of rest; for nature also is in the same genus as potentiality, because it is a principle of motion, although not in some other thing, but in the thing itself qua itself.⁵ To every potentiality of this kind actuality is prior, both in formula and in substance; in time it is sometimes prior and sometimes not. That actuality is prior in formula is evident; for it is because it can be actualized that the potential, in the primary sense, is potential, I mean, e.g., that the potentially constructive is that which can construct, the potentially seeing that which can see, and the potentially visible that which can be seen. The same principle holds in all other cases too, so that the formula and knowledge of the actual must precede the knowledge of the potential. In time it is prior in this sense: the actual is prior to the potential with which it is formally identical, but not to that with which it is identical numerically. What I mean is this: that the matter and the seed and the thing which is capable of seeing, which are potentially a man and corn and seeing, but are not yet so actually, are prior in time to the individual man and corn and seeing subject which already exist in actuality. But prior in time to these potential entities are other actual entities from which the former are generated; for the actually existent is always generated from the potentially existent by something which is actually existent—e.g., man by man, cultured by cultured—there is always some prime mover; and that which initiates motion exists already in actuality. We have said⁶ in our discussion of substance that everything which is generated is generated from something and by something; and by something formally identical with itself. Hence it seems impossible that a man can be a builder if he has never built, or a harpist if he has never played a harp; because he who learns to play the harp learns by playing it, and similarly in all other cases. This was the origin of the sophists’ quibble that a man who does not know a given science will be doing that which is the object of that science, because the learner does not know the science. But since something of that which is being generated is already generated, and something of that which is being moved as a whole is already moved (this is demonstrated in our discussion on Motion⁷), 1050apresumably the learner too must possess something of the science. At any rate from this argument it is clear that actuality is prior to potentiality in this sense too, i.e. in respect of generation and time. But it is also prior in substantiality; (a) because things which are posterior in generation are prior in form and substantiality; e.g., adult is prior to child, and man to semen, because the one already possesses the form, but the other does not; and (b) because everything which is generated moves towards a principle, i.e. its end . For the object of a thing is its principle; and generation has as its object the end . And the actuality is the end, and it is for the sake of this that the potentiality is acquired; for animals do not see in order that they may have sight, but have sight in order that they may see. Similarly men possess the art of building in order that they may build, and the power of speculation that they may speculate; they do not speculate in order that they may have the power of speculation—except those who are learning by practice; and they do not really speculate, but only in a limited sense, or about a subject about which they have no desire to speculate. Further, matter exists potentially, because it may attain to the form; but when it exists actually, it is then in the form. The same applies in all other cases, including those where the end is motion. Hence, just as teachers think that they have achieved their end when they have exhibited their pupil performing, so it is with nature. For if this is not so, it will be another case of “Pauson’s Hermes”⁸; it will be impossible to say whether the knowledge is in the pupil or outside him, as in the case of the Hermes. For the activity is the end, and the actuality is the activity; hence the term “actuality” is derived from “activity,” and tends to have the meaning of “complete reality.” Now whereas in some cases the ultimate thing is the use of the faculty, as, e.g., in the case of sight seeing is the ultimate thing, and sight produces nothing else besides this; but in other cases something is produced, e.g. the art of building produces not only the act of building but a house; nevertheless in the one case the use of the faculty is the end, and in the other it is more truly the end than is the potentiality. For the act of building resides in the thing built; i.e., it comes to be and exists simultaneously with the house. Thus in all cases where the result is something other than the exercise of the faculty, the actuality resides in the thing produced; e.g. the act of building in the thing built, the act of weaving in the thing woven, and so on; and in general the motion resides in the thing moved. But where there is no other result besides the actualization, the actualization resides in the subject; e.g. seeing in the seer, and speculation in the speculator, and life in the soul 1050b(and hence also happiness, since happiness is a particular kind of life). Evidently, therefore, substance or form is actuality. Thus it is obvious by this argument that actuality is prior in substantiality to potentiality; and that in point of time, as we have said, one actuality presupposes another right back to that of the prime mover in each case. It is also prior in a deeper sense; because that which is eternal is prior in substantiality to that which is perishable, and nothing eternal is potential. The argument is as follows. Every potentiality is at the same time a potentiality for the opposite.⁹ For whereas that which is incapable of happening cannot happen to anything, everything which is capable may fail to be actualized. Therefore that which is capable of being may both be and not be. Therefore the same thing is capable both of being and of not being. But that which is capable of not being may possibly not be; and that which may possibly not be is perishable; either absolutely, or in the particular sense in which it is said that it may possibly not be; that is, in respect either of place or of quantity or of quality. “Absolutely” means in respect of substance. Hence nothing which is absolutely imperishable is absolutely potential (although there is no reason why it should not be potential in some particular respect; e.g. of quality or place); therefore all imperishable things are actual. Nor can anything which is of necessity be potential; and yet these things are primary, for if they did not exist, nothing would exist. Nor can motion be potential, if there is any eternal motion. Nor, if there is anything eternally in motion, is it potentially in motion (except in respect of some starting-point or destination), and there is no reason why the matter of such a thing should not exist. Hence the sun and stars and the whole visible heaven are always active, and there is no fear that they will ever stop—a fear which the writers¹⁰ on physics entertain. Nor do the heavenly bodies tire in their activity; for motion does not imply for them, as it does for perishable things, the potentiality for the opposite, which makes the continuity of the motion distressing; this results when the substance is matter and potentiality, not actuality. Imperishable things are resembled in this respect by things which are always undergoing transformation, such as earth and fire; for the latter too are always active, since they have their motion independently and in themselves.¹¹ Other potentialities, according to the distinctions already made,¹² all admit of the opposite result; for that which is capable of causing motion in a certain way can also cause it not in that way; that is if it acts rationally. The same irrational potentialities can only produce opposite results by their presence or absence. Thus if there are any entities or substances such as the dialecticians¹³ describe the Ideas to be, there must be something which has much more knowledge than absolute knowledge, and much more mobility than motion; 1051afor they will be in a truer sense actualities, whereas knowledge and motion will be their potentialities.¹⁴ Thus it is obvious that actuality is prior both to potentiality and to every principle of change.