Lecture 14 — “Potency,” “capable,” “incapable,” “possible,” and “impossible”

In one sense the term “potency” or “power” means the principle of motion or change in some other thing as other; for example, the art of building is a potency which is not present in the thing built; but the art of medicine is a potency and is present in the one healed, but not inasmuch as he is healed. In general, then, “potency” means the principle of change or motion in some other thing as other.

Or it means the principle of a thing’s being moved or changed by some other thing as other.

For, by reason of that principle by which a patient undergoes some change, we sometimes say that it has the potency of passion if it is possible for it to undergo any change at all.

But sometimes we do not say this by reason of every change which a thing can undergo, but only if the change is for the better.

And in another sense, “potency” means the ability or power to do this particular thing well or according to intention. For sometimes we say of those who can merely walk or talk but not well or as they planned that they cannot walk or talk. And the same applies to things that are undergoing change.

Further, all states in virtue of which things are altogether unsusceptible to change or immutable, or are not easily changed for the worse, are called “potencies” or “powers.”

For things are broken and crushed and bent and in general destroyed not because they have a potency, but because they do not have one and are deficient in some way.

And things are not susceptible to such processes when they are hardly or slightly affected by them because they have the potency and the ability to be in some definite state.

And since the term “potency” is used in these senses, the term “capable” or “potent” will be used in the same number of senses.

Thus, in one sense, whatever has within itself the source of the motion or change which takes place in some other thing as other (for even something that brings another to rest is potent in a sense) is said to be capable. And in another sense, that which receives such a potency or power from it is said to be capable.

And in still another sense a thing is said to be capable if it has the potency of being changed in some way, whether for the worse or for the better. For anything which is corrupted seems to be capable of being corrupted, since it would not have been corrupted if it had been incapable of it.

But as matters stand, it already has a certain disposition and cause and principle to undergo such change. Hence sometimes a thing seems to be such (that is, capable) because it has something, and sometimes because it is deprived of something.

But if privation is in a sense a having, all things will be capable or potent by having something. But “being” is used in two different senses. Hence a thing is capable both by having some privation and principle and by having the privation of this, if it can have a privation.

And in another sense a thing is capable because there is no potency or power in some other thing as other which can corrupt it.

Again, all these things are capable either because they merely might happen to come into being or not, or because they might do so well.

For this sort of potency or power is found in inanimate things such as instruments. For men say that one lyre can produce a sound, and that another cannot, if it does not have a good tone.

Incapacity, on the other hand, is a privation of capacity, a kind of removal of such a principle as has been described, either altogether, or in the case of something which is naturally disposed to have it, or when it is already naturally disposed to have it and does not. For it is not in the same way that a boy, a man, and a eunuch are said to be incapable of begetting.

Again, there is an incapacity corresponding to each kind of capacity, both to that which can merely produce motion and to that which can produce it well.

And some things are said to be incapable according to this sense of “incapacity,” but others in a different sense, namely, as possible and impossible.

“Impossible” means that of which the contrary is necessarily true; thus it is impossible that the diagonal of a square should be commensurable with a side, because such a statement is false of which the contrary is not only true but also necessarily so—that is, that the diagonal is not commensurable.

Therefore, that the diagonal is commensurable is not only false, but necessarily false.

And the contrary of this—the possible—is when the contrary is not necessarily false. For example, it is possible that a man should be seated, because it is not necessarily false that he should not be seated. Hence the term “possible” means, in one sense (as has been stated), whatever is not necessarily false; and in another sense, whatever is true; and in still another, whatever may be true.

And what is called “a power” in geometry is called such metaphorically. These senses of “capable,” then, do not refer to potency.

But those senses which do refer to potency are all used in reference to the one primary sense of potency, namely, a principle of change in some other thing inasmuch as it is other.

And other things are said to be “capable” in a passive sense—some because some other thing has such power over them, some because it does not, and some because it has it in a special way. The same applies to the term “incapable.”

Hence the proper definition of the primary kind of potency will be a principle of change in some other thing as other.

“Potency/capacity”

954. Having treated the various senses of the terms which signify the parts of unity, here Aristotle begins to treat those which signify the parts of being.

He does this first according as being is divided by act and potency;

second (1020a7; [977]), according as it is divided by the ten categories, at “quantity” means.

In regard to the first, he gives the various senses in which the term “potency” or “power” is used. But he omits the term “act,” because he could explain its meaning adequately only if the nature of forms had been made clear first, and he will do this in books 8 [1703] and 9 [1823]. Hence in book 9 he immediately settles the question about potency and act together. This part, then, is divided into two members.

In the first he explains the various senses in which the term “potency” is used;

in the second (1019b35; [975]), he reduces all of them to one primary sense, at but those senses.

In regard to the first he does two things.

First, he gives the various senses in which the term “potency” is used;

and second (1019b15; [967]), the various senses in which the term incapacity is used, at incapacity, on the other hand.

In treating the first he does two things.

First, he gives the senses in which the term “potency” is used;

second (1019a32; [961]), those in which the term capable or potent is used, at and since the term.

955. In dealing with the first part, then, he gives four senses in which the term “potency” or “power” is used.

First, “potency” means an active principle of motion or change in some other thing as other. For there is some principle of motion or change in the thing changed, namely, the matter, or some formal principle on which the motion depends, as upward or downward motion is a result of the forms of lightness or heaviness. But a principle of this kind cannot be designated as the active power on which this motion depends. For everything which is moved is moved by another, and a thing moves itself only by means of its parts inasmuch as one part moves another, as is proved in Physics 8.

Hence, insofar as a potency is a principle of motion in the thing where motion is found, it is not included under active power, but under passive potency. For heaviness in earth is not a principle causing motion, but rather one which causes it to be moved. Hence active power must be present in some other thing than the one moved—for example, the power of building is not in the thing being built but rather in the builder. And while the art of medicine is an active power, since the physician heals by means of it, it may also be found in the one who is healed not inasmuch as he is healed, but accidentally, inasmuch as the physician and the one who is healed happen to be the same. So therefore, generally speaking, “potency” or “power” means in one sense a principle of motion or change in some other thing as other.

956. Or it means (1019a20). Here he gives a second sense in which the term “potency” is used. He says that in another sense the term “potency” means the principle whereby something is moved or changed by another thing as other. Now this is passive potency, and it is by reason of it that a patient undergoes some change.

For just as every agent or mover moves something other than itself and acts in something other than itself, so too every patient is acted upon by something other than itself—that is, everything moved is moved by another. For that principle whereby one thing is properly moved or acted upon by another is called “passive potency.”

957. Now there are two ways in which we can say that a thing has the potency to be acted upon by another.

Sometimes we attribute such a potency to something, whatever it may be, because it is able to undergo some change, whether it be good or bad.

And sometimes we say that a thing has such a potency not because it can undergo something evil, but because it can be changed for the better.

For example, we do not say that one who can be overpowered has a potency in this last sense, but we do attribute such a potency to one who can be taught or helped. And we speak thus because sometimes an ability to be changed for the worse is attributed to incapacity, and the ability not to be changed in the same way is attributed to potency, as will be said below [965].

958. Another text reads, sometimes this is not said of every change which a thing undergoes but of change to a contrary. This should be understood thus: whatever receives a perfection from something else is said in an improper sense to undergo a change, and it is in this sense that to understand is said to be a kind of passion. But that which receives along with a change in itself something other than what is natural to it is said to undergo a change in a proper sense.

Hence such passion is also said to be a removing of something from a substance. But this can come about only by way of some contrary. Therefore, when a thing is acted upon in a way contrary to its own nature or condition, it is said in a proper sense to undergo a change or to be passive. And in this sense even illnesses are called passions. But when a thing receives something which is fitting to it by reason of its nature, it is said to be perfected rather than passive.

959. And in another sense (1019a23). He now gives a third sense in which the term “potency” is used. He says that in another sense “potency” means the principle of performing some act, not in any way at all, but well or according to intention, that is, according to what a man plans. For when men walk or talk but not well or as they planned to do, we say that they do not have the ability to walk or to talk.

And the same applies to things that are undergoing change, for a thing is said to be able to undergo something if it can undergo it well.

For example, some pieces of wood are said to be combustible because they can be burned easily, and others are said to be incombustible because they cannot be burned easily.

960. Further, all states (1019a26). He gives a fourth sense in which the term “potency” is used. He says that we designate as potencies all habits or forms or dispositions by which some things are said or made to be altogether incapable of being acted upon or changed, or to be not easily changed for the worse.

For when bodies are changed for the worse, as those which are broken or bent or crushed or destroyed in any way at all, this does not happen to them because of some ability or potency, but rather because of some inability and the weakness of some principle which does not have the power of resisting the thing which destroys them. For a thing is destroyed only because of the victory which the destroyer wins over it, and this is a result of the weakness of its proper active power.

For those things that cannot be affected by defects of this kind, or can hardly or slightly be affected by them (that is, they are affected slowly or to a small degree) are such because they have the potency and the ability to be in some definite state; that is, they have a certain perfection which prevents them from being overcome by contraries.

And, as is said in the Categories, it is in this way that “hard” or “healthy” signifies a natural power which a thing has of resisting change by destructive agents. But “soft” and “sickly” signify incapacity or lack of power.

961. And since the term (1019a32). Here he gives the senses of the term “capable” or “potent” which correspond to the above senses of “potency.” And there are two senses of “capable” which correspond to the first sense of “potency.”

For according to its active power, a thing is said to be capable of acting in two ways: in one way, because it acts immediately of itself; in another way, because it acts through something else to which it communicates its power, as a king acts through a bailiff.

Hence he says that, since the term “potency” is used in this number of senses, the term “capable” or “potent” must also be used in the same number of senses. Thus in one sense it means something which has an active principle of change in itself, as what brings another to rest or to a stop; that is, what causes some other thing to stand still is said to be capable of bringing something different from itself to a state of rest. And it is used in another sense when a thing does not act directly, but another thing receives such power from it that it can act directly.

962. And in still another (1019b1). Next, he gives a second sense in which the term “capable” is used, and this corresponds to the second sense of the term “potency,” namely, passive potency. He says that, in a different way from the foregoing, a thing is said to be capable or potent when it can be changed in some respect, whatever it may be, that is, whether it can be changed for the better or for the worse.

And in this sense a thing is said to be corruptible because it is capable of being corrupted, which is to undergo change for the worse, or it is not corruptible because it is capable of not being corrupted, assuming that it is impossible for it to be corrupted.

963. And what is capable of being acted upon in some way must have within itself a certain disposition which is the cause and principle of its passivity, and this principle is called “passive potency.”

But such a principle can be present in the thing acted upon for two reasons.

First, this is because it possesses something; for example, a man is capable of suffering from some disease because he has an excessive amount of some inordinate humor.

Second, a thing is capable of being acted upon because it lacks something which could resist the change. This is the case, for example, when a man is said to be capable of suffering from some disease because his strength and natural power have been weakened.

Now both of these must be present in anything which is capable of being acted upon, for a thing would never be acted upon unless it both contained a subject which could receive the disposition or form induced in it as a result of the change and also lacked the power of resisting the action of an agent.

964. Now these two ways in which the principle of passivity is spoken of can be reduced to one, because privation can be designated as “a having.” Thus it follows that to lack something is to have a privation, and so each way will involve the having of something. Now the designation of privation as a having and as something had follows from the fact that “being” is used in two different ways; and both privation and negation are called being in one of these ways, as has been pointed out at the beginning of book 4 [564]. Hence it follows that negation and privation can also be designated as “havings.” We can say, then, that in general something is capable of passion because it contains a kind of “having” and a certain principle that enables it to be acted upon—for even to lack something is to have something, if a thing can have a privation.

965. An in another sense (1019b10). Here he gives a third sense in which the term “capable” is used, and this sense corresponds to the fourth sense of “potency” inasmuch as a potency was said to be present in something which cannot be corrupted or changed for the worse. Thus he says that in another sense a thing is said to be capable because it does not have some potency or principle which enables it to be corrupted. And I mean by some other thing as other. For a thing is said to be potent or powerful in the sense that it cannot be overcome by something external so as to be corrupted.

966. Again, all these (1019b11). He gives a fourth sense in which the term “capable” is used, and this corresponds to the third sense of “potency” inasmuch as “potency” designated the ability to act or be acted upon well. He says that according to the foregoing senses of potency which pertain both to acting and to being acted upon, a thing can be said to be capable either because it merely happens to come into being or not or because it happens to come into being well.

For a thing is said to be capable of acting either because it can simply act or because it can act well and easily. And in a similar way a thing is said to be capable of being acted upon and corrupted because it can be acted upon easily.

And this sense of “potency” is also found in inanimate things such as instruments, in the case of the lyre and other musical instruments. For one lyre is said to be able to produce a tone because it has a good tone, and another is said not to because its tone is not good.

“Incapacity”

967. Incapacity (1019b15). Then he gives the different senses of the term “incapacity,” and in regard to this he does two things.

First, he gives the various senses in which we speak of incapacity;

second (1019b21; [970]), he treats the different senses in which the term “impossible” is used, at and some things.

In treating the first part he does two things.

First, he gives the common meaning of the term “incapacity.”

Second (1019b19; [969]), he notes the various ways in which it is used, at again, there is.

He accordingly says, first, that incapacity is the privation of potency. Now two things are required in the notion of privation, and the first of these is the removal of an opposite state. But the opposite of incapacity is potency. Therefore, since potency is a kind of principle, incapacity will be the removal of that kind of principle which potency has been described to be.

The second thing required is that privation properly speaking must belong to a definite subject and at a definite time; and it is taken in an improper sense when taken without a definite subject and without a definite time. For, properly speaking, only that is said to be blind which is naturally fitted to have sight and at the time when it is naturally fitted to have it.

968. And he says that incapacity, such as it has been described, is the removal of a potency: either altogether, that is, universally, in the sense that every removal of a potency is called incapacity, whether the thing is naturally disposed to have the potency or not; or from something which is naturally fitted to have it at some time or other; or only at the time when it is naturally fitted to have it.

For incapacity is not taken in the same way when we say that a boy is incapable of begetting and when we say this of a man and of a eunuch. For to say that a boy is incapable of begetting means that, while the subject is naturally fitted to beget, it cannot beget before the proper time. But to say that a eunuch is incapable of begetting means that, while he was naturally fitted to beget at the proper time, he cannot beget now, for he lacks the active principles of begetting. Hence incapacity here retains the notion of privation. But a mule or a stone is said to be incapable of begetting because neither can do so, and also because neither has any real aptitude for doing so.

969. Again, there is (1019b19). Then he explains the various senses of “incapacity” by contrasting them with the senses of “potency.” For just as potency is twofold—active and passive—and both refer either to acting and being acted upon simply, or to acting and being acted upon well, in a similar fashion there is an opposite sense of “incapacity” corresponding to each type of potency.

That is to say, there is a sense of “incapacity” corresponding both to that which can merely produce motion and to that which can produce it well, namely, to active potency, which is the potency to simply move a thing or to move it well, and to passive potency, which is the potency to simply be moved or to be moved well.

970. And some things (1019b21). Then he explains the various senses in which the term “impossible” is used, and in regard to this he does two things.

First, he gives the various senses in which the term “impossible” is used;

second (1019b35; [975]), he reduces them to one, at but those senses.

In regard to the first he does three things.

First, he says that in one sense some things are said to be impossible because they have the foregoing incapacity which is opposed to potency. And “impossible” in this sense is used in four ways corresponding to those of “incapacity.”

971. Accordingly, when he says in a different sense, he gives another way in which some things are said to be impossible. And they are said to be such not because of the privation of some potency, but because of the opposition existing between the terms in propositions. For since potency is referred to being, then just as being is predicated not only of things that exist in reality but also of the composition of a proposition inasmuch as it contains truth and falsity, in a similar fashion the terms “possible” and “impossible” are predicated not only of real potency and incapacity, but also of the truth and falsity found in the combining or separating of terms in propositions. Hence the term “impossible” means that of which the contrary is necessarily true. For example, it is impossible that the diagonal of a square should be commensurable with a side, because such a statement is false whose contrary is not only true, but necessarily so—namely, that it is not commensurable. Hence the statement that it is commensurable is necessarily false. And this is impossible.

972. And the contrary (1019b27). Here he shows that the possible is the opposite of the impossible in the second way mentioned, for the impossible is opposed to the possible in the second way mentioned. He says, then, that “the possible,” as the contrary of this second sense of “the impossible,” means that whose contrary is not necessarily false. For example, it is possible that a man should be seated, because the opposite of this—that he should not be seated—is not necessarily false.

973. From this it is clear that this sense of “possible” has three usages.

For in one way it designates what is false but is not necessarily so—for example, it is possible that a man should be seated while he is not seated, because the opposite of this is not necessarily true.

In another way, “possible” designates what is true but is not necessarily so because its opposite is not necessarily false—for example, that Socrates should be seated while he is seated.

And in a third way it means that, although a thing is not true now, it may be true later on.

974. And what is called a “power” (1019b33). He shows how the term “power” is used metaphorically. He says that in geometry the term “power” is used metaphorically. For in geometry the square of a line is called its “power” by reason of the following likeness, namely, that just as from something in potency something actual comes to be, in a similar way, from multiplying a line by itself, its square results.

It would be the same if we were to say that the number three is capable of becoming the number nine, because from multiplying the number three by itself the number nine results, for three times three makes nine. And just as the term “impossible” taken in the second sense does not correspond to any incapacity, in a similar way the senses of the term “possible” which were given last do not correspond to any potency, but they are used figuratively or in the sense of the true and the false.

975. But those senses (1019b35). He now reduces all senses of “capable” and “incapable” to one primary sense. He says that those senses of the term “capable” or “potent” which correspond to potency all refer to one primary kind of potency—the first active potency which was described above [955] as the principle of change in some other thing as other, because all the other senses of “capable” or “potent” are referred to this kind of potency. For a thing is said to be capable by reason of the fact that some other thing has active power over it, and in this sense it is said to be capable according to passive potency. And some things are said to be capable because some other thing does not have power over them, as those which are said to be capable because they cannot be corrupted by external agents. And others are said to be capable because they have it in some special way, that is, because they have the power or potency to act or be acted upon well or easily.

976. And just as all things that are said to be capable because of some potency are reduced to one primary potency, in a similar way all things that are said to be incapable because of some impotency are reduced to one primary incapacity, which is the opposite of the primary potency. It is clear, then, that the proper notion of potency in the primary sense is this: a principle of change in some other thing as other. This is the notion of active potency or power.