1045bWe have now dealt with Being in the primary sense, to which all the other categories of being are related; i.e. substance. For it is from the concept of substance that all the other modes of being take their meaning; both quantity and quality and all other such terms; for they will all involve the concept of substance, as we stated it in the beginning of our discussion.¹ And since the senses of being are analyzable² not only into substance or quality or quantity, but also in accordance with potentiality and actuality and function, let us also gain a clear understanding about potentiality and actuality; and first about potentiality in the sense which is most proper to the word, but not most useful for our present purpose— 1046afor potentiality and actuality extend beyond the sphere of terms which only refer to motion. When we have discussed this sense of potentiality we will, in the course of our definitions of actuality,³ explain the others also. We have made it plain elsewhere⁴ that “potentiality” and “can” have several senses. All senses which are merely equivocal may be dismissed; for some are used by analogy, as in geometry,⁵ and we call things possible or impossible because they “are” or “are not” in some particular way. But the potentialities which conform to the same type are all principles, and derive their meaning from one primary sense of potency, which is the source of change in some other thing, or in the same thing qua other. One kind of potentiality is the power of being affected; the principle in the patient itself which initiates a passive change in it by the action of some other thing, or of itself qua other. Another is a positive state of impassivity in respect of deterioration or destruction by something else or by itself qua something else; i.e. by a transformatory principle—for all these definitions contain the formula of the primary sense of potentiality. Again, all these potentialities are so called either because they merely act or are acted upon in a particular way, or because they do so well . Hence in their formulae also the formulae of potentiality in the senses previously described are present in some degree. Clearly, then, in one sense the potentiality for acting and being acted upon is one (for a thing is “capable” both because it itself possesses the power of being acted upon, and also because something else has the power of being acted upon by it); and in another sense it is not; for it is partly in the patient (for it is because it contains a certain principle, and because even the matter is a kind of principle, that the patient is acted upon; i.e., one thing is acted upon by another: oily stuff is inflammable, and stuff which yields in a certain way is breakable, and similarly in other cases)— and partly in the agent; e.g. heat and the art of building: the former in that which produces heat, and the latter in that which builds. Hence in so far as it is a natural unity, nothing is acted upon by itself; because it is one, and not a separate thing. “Incapacity” and “the incapable” is the privation contrary to “capacity” in this sense; so that every “capacity” has a contrary incapacity for producing the same result in respect of the same subject. Privation has several senses⁶—it is applied (1.) to anything which does not possess a certain attribute; (2.) to that which would naturally possess it, but does not; either (a) in general, or (b) when it would naturally possess it; and either (1) in a particular way, e.g. entirely, or (2) in any way at all. And in some cases if things which would naturally possess some attribute lack it as the result of constraint, we say that they are “deprived.”

Since some of these principles are inherent in inanimate things, and others in animate things and in the soul and in the rational part of the soul, 1046bit is clear that some of the potencies also will be irrational and some rational. Hence all arts, i.e. the productive sciences, are potencies; because they are principles of change in another thing, or in the artist himself qua other. Every rational potency admits equally of contrary results, but irrational potencies admit of one result only. E.g., heat can only produce heat, but medical science can produce disease and health. The reason of this is that science is a rational account, and the same account explains both the thing and its privation, though not in the same way; and in one sense it applies to both, and in another sense rather to the actual fact. Therefore such sciences must treat of contraries—essentially of the one, and non-essentially of the other; for the rational account also applies essentially to the one, but to the other in a kind of accidental way, since it is by negation and removal that it throws light on the contrary. For the contrary is the primary privation,⁷ and this is the removal of that to which it is contrary.⁸ And since contrary attributes cannot be induced in the same subject, and science is a potency which depends upon the possession of a rational formula, and the soul contains a principle of motion, it follows that whereas “the salutary” can only produce health, and “the calefactory” only heat, and “the frigorific” only cold, the scientific man can produce both contrary results. For the rational account includes both, though not in the same way; and it is in the soul, which contains a principle of motion, and will therefore, by means of the same principle, set both processes in motion, by linking them with the same rational account. Hence things which have a rational potency produce results contrary to those of things whose potency is irrational⁹; for the results of the former are included under one principle, the rational account. It is evident also that whereas the power of merely producing (or suffering) a given effect is implied in the power of producing that effect well , the contrary is not always true; for that which produces an effect well must also produce it, but that which merely produces a given effect does not necessarily produce it well.

There are some, e.g. the Megaric school,¹⁰ who say that a thing only has potency when it functions, and that when it is not functioning it has no potency. E.g., they say that a man who is not building cannot build, but only the man who is building, and at the moment when he is building; and similarly in the other cases. It is not difficult to see the absurd consequences of this theory. Obviously a man will not be a builder unless he is building, because “to be a builder” is “to be capable of building”; and the same will be true of the other arts. If, therefore, it is impossible to possess these arts without learning them at some time and having grasped them, 1047aand impossible not to possess them without having lost them at some time (through forgetfulness or some affection or the lapse of time; not, of course, through the destruction of the object of the art,¹¹ because it exists always), when the artist ceases to practice his art, he will not possess it; and if he immediately starts building again, how will he have re-acquired the art? The same is true of inanimate things. Neither the cold nor the hot nor the sweet nor in general any sensible thing will exist unless we are perceiving it (and so the result will be that they are affirming Protagoras’ theory¹²). Indeed, nothing will have the faculty of sensation unless it is perceiving, i.e. actually employing the faculty. If, then, that is blind which has not sight, though it would naturally have it, and when it would naturally have it, and while it still exists, the same people will be blind many times a day; and deaf too. Further, if that which is deprived of its potency is incapable, that which is not happening will be incapable of happening; and he who says that that which is incapable of happening is or will be, will be in error, for this is what “incapable” meant.¹³ Thus these theories do away with both motion and generation; for that which is standing will always stand, and that which is sitting will always sit; because if it is sitting it will not get up, since it is impossible that anything which is incapable of getting up should get up. Since, then, we cannot maintain this, obviously potentiality and actuality are different. But these theories make potentiality and actuality identical; hence it is no small thing that they are trying to abolish. Thus it is possible that a thing may be capable of being and yet not be, and capable of not being and yet be; and similarly in the other categories that which is capable of walking may not walk, and that which is capable of not walking may walk. A thing is capable of doing something if there is nothing impossible in its having the actuality of that of which it is said to have the potentiality. I mean, e.g., that if a thing is capable of sitting and is not prevented from sitting, there is nothing impossible in its actually sitting; and similarly if it is capable of being moved or moving or standing or making to stand or being or becoming or not being or not becoming. The term “actuality,” with its implication of “complete reality,” has been extended from motions, to which it properly belongs, to other things; for it is agreed that actuality is properly motion. Hence people do not invest non-existent things with motion, although they do invest them with certain other predicates. E.g., they say that non-existent things are conceivable and desirable, but not that they are in motion. This is because, although these things do not exist actually, they will exist actually; 1047bfor some non-existent things exist potentially; yet they do not exist, because they do not exist in complete reality.