Section 12

Potency 1 means: (a) the source of motion or change which is in something other than the thing changed, or in it qua other. E.g., the science of building is a potency which is not present in the thing built; but the science of medicine, which is a potency, may be present in the patient, although not qua patient.

Thus potency means the source in general of change or motion in another thing, or in the same thing qua other; or the source of a thing’s being moved or changed by another thing, or by itself qua other (for in virtue of that principle by which the passive thing is affected in any way we call it capable of being affected; sometimes if it is affected at all, and sometimes not in respect of every affection, but only if it is changed for the better).

(b) The power of performing this well or according to intention; because sometimes we say that those who can merely take a walk, or speak, without doing it as well as they intended, cannot speak or walk. And similarly in the case of passivity.

(c) All states in virtue of which things are unaffected generally, or are unchangeable, or cannot readily deteriorate, are called potencies. For things are broken and worn out and bent and in general destroyed not through potency but through impotence and deficiency of some sort; and things are unaffected by such processes which are scarcely or slightly affected because they have a potency and are potent and are in a definite state.

Since potency has all these meanings, potent (or capable) will mean (a) that which contains a source of motion or change (for even what is static is potent in a sense) which takes place in another thing, or in itself qua other. (b) That over which something else has a potency of this kind. (c) That which has the potency of changing things, either for the worse or for the better (for it seems that even that which perishes is capable of perishing; otherwise, if it had been incapable, it would not have perished. As it is, it has a kind of disposition or cause or principle which induces such an affection.

Sometimes it seems to be such as it is because it has something, and sometimes because it is deprived of something; but if privation is in a sense a state or habit, everything will be potent through having something; and so a thing is potent in virtue of having a certain habit or principle, and also in virtue of having the privation of that habit, if it can have privation; and if privation is not in a sense habit, the term potent is equivocal).

(d) A thing is potent if neither any other thing nor itself qua other contains a potency or principle destructive of it. (e) All these things are potent either because they merely might chance to happen or not to happen, or because they might do so well . Even in inanimate things this kind of potency is found; e.g. in instruments; for they say that one lyre can be played, and another not at all, if it has not a good tone.

Impotence is a privation of potency—a kind of abolition of the principle which has been described—either in general or in something which would naturally possess that principle, or even at a time when it would naturally already possess it (for we should not use impotence—in respect of begetting—in the same sense of a boy, a man and a eunuch). Again, there is an impotence corresponding to each kind of potency; both to the kinetic and to the successfully kinetic.

Some things are said to be impotent in accordance with this meaning of impotence, but others in a different sense, namely possible and impossible. Impossible means: (a) that whose contrary is necessarily true; e.g., it is impossible that the diagonal of a square should be commensurable with the sides, because such a thing is a lie, whose contrary is not only true but inevitable. Hence that it is commensurable is not only a lie but necessarily a lie.

And the contrary of the impossible, i.e. the possible, is when the contrary is not necessarily a lie; e.g., it is possible that a man should be seated, for it is not necessarily a lie that he should not be seated. Possible, then, means in one sense, as we have said, that which is not necessarily a lie; in another, that which is true; and in another, that which may be true.

(The power in geometry 2 is so called by an extension of meaning.)

These are the senses of potent which do not correspond to potency. Those which do correspond to it all refer to the first meaning, i.e. a source of change which exists in something other than that in which the change takes place, or in the same thing qua other.

Other things are said to be potent 3 because something else has such a potency over them; others because it does not possess it; others because it possesses it in a particular way. The term impotent is similarly used. Thus the authoritative definition of potency in the primary sense will be a principle producing change, which is in something other than that in which the change takes place, or in the same thing qua other.