Lecture 10 — Potency and act in sensation

These questions being settled, let us speak of sensation in general. As has been said, sensation occurs in something being moved and acted upon; for it appears to be a certain alteration. Now, some say, ‘like is acted upon by like.’ In what way this is possible or impossible has been stated in our universal accounts about acting and being acted upon.

It may be asked why there is no sensation of the senses themselves and why they do not produce sensation without something extraneous, seeing that they contain within themselves fire and earth and the other elements that the senses concern, either of themselves or according to their accidents.

It becomes evident, then, that the sensitive power is not in act but is only in potency, which explains why it does not sense, just as the combustible does not burn of itself without something to make it burn. Otherwise it would burn itself, and not need an actual fire to be present.

But as we speak of sensing in two ways (for we say that one who sees and hears in potency sees and hears, whether he happens to be asleep or is actively doing it), so we may speak of sense in two ways, as in potency and as in act. Likewise, to sense is also both a being in potency and in act.

To start with, then, let us speak as if being acted upon and being moved were the same as acting. For even movement is a certain act, though imperfect, as has been stated elsewhere.

All things are moved and acted upon by an agent and a being in act. Hence it is that a thing is acted upon both by its like and also by its unlike, as we have said. For what is being acted upon is unlike; what has been acted upon is like.

350. These questions being settled (416b32). After treating of the vegetative part of the soul, the Philosopher now begins to examine the sensitive part. This treatment divides into two sections.

The first section deals with what is most apparent in the sensitive part, that is, the exterior senses.

The second section, beginning at that there is no other sense (424b22; [564]), treats of what is hidden therein.

The first is divided into two parts.

In the first, Aristotle explains how senses are related to sensible objects.

In the second, at in treating of each sense (418a7; [383]), he defines both the sensible object and the sense.

About the first, he does two things:

first, he repeats some earlier observations;

second, at it may be asked (417a2; [352]), he proceeds to the present problem.

First of all, then, he remarks that these questions being settled that pertain to the vegetative part, he must now speak with a general view of sense, going on later to a more detailed treatment of each sense in particular. (Lect. 14–24.) And he repeats two things already said: that to sense consists in being moved or acted upon, for the act of sensation involves a certain alteration, which is being altered, acted upon, and moved; and second, he takes up the fact that some say ‘like is acted upon by like’, and this is because sensation is a certain being acted upon:

351. so some early thinkers held that like is known and sensed by like: as Empedocles said that earth knows earth, fire knows fire, and so on.

How it can be so (and not so) that like is acted on by like has been stated in our universal accounts of acting and being acted upon, that is, in On Generation, where he considers acting and being acted upon in common. For he says there that, although what is acted upon at the beginning, while it is being acted upon, is contrary to the agent, at the end, when it has already been acted upon, it is like it. (Aristotle, On Generation and Corruption 1.7, 323b1–324b24; cf. Aquinas, Commentary on Aristotle’s On Generation and Corruption, bk. 1, lect. 19.) For an agent, in acting, makes the patient like himself.

352. Then, at it may be asked (417a2), he proceeds to determine the truth of the present problem. And about this he does three things.

First, he shows that in themselves the senses are in potency;

second, at but as we speak of sensing in two ways (417a10; [355]), that they are sometimes in act;

third, at distinctions, however (417a21; [358]), he shows how they are reduced from potency into act.

To understand the first, note that all who, like Empedocles, said that like is known by like thought that the senses were the actual sense-objects themselves; for he asserted that the sensitive soul, to know all sense-objects, must in some way be composed of all things sensible, insofar as it was put together (according to them) from the elements of sensible things.

353. Two things follow from this hypothesis.

One is that, if the senses actually are, or are made up of, the sense-objects themselves, then, if the actual sensible things themselves can be sensed, the senses themselves can be sensed.

The second is that, since a sense can sense when the sensible objects are present, then, if this object actually exists in the sense as part of its composition, it follows that a sense can sense in the absence of external sensible objects.

But both these things are false. He introduces these two incongruities that follow from the ancient view under the form of a question as things that the early philosophers could not solve. So he says, it may be asked why there is no sensation of the senses, that is, why the senses themselves are not sensed; for it seems they would be sensed if the senses are like the sensible objects.

354. It is also hard to see why they do not produce sensation, that is, why actual sensation does not occur, without something extraneous, that is, without exterior sense-objects, since existing internal to the senses themselves are, in the opinion of the ancients, fire, earth, and the other elements, which are sensible either in themselves, that is, in their own substance, according to those who did not discern between the senses and the intellect (for intellect is properly able to know substance), or according to their proper accidents, namely, hot and cold and so forth, which are sensible through themselves. Now, since these difficulties are insurmountable if the sense power has its objects in act (as the early philosophers thought), Aristotle concludes it is evident that the sensitive soul is the sense-object not in act but is only in potency. That is why the sense does not sense without an exterior sense-object, just as the combustible does not burn of itself without an exterior agent to make it burn; whereas if it were actually fire, it would burn of itself.

355. Then, at but as we speak of sensing (417a10), he shows that sensation is sometimes in act. About this, he does three things.

First, he shows that sensation is sometimes in act in two ways in which we speak of anyone sensing. For we sometimes say that someone sees and hears when he only sees and hears potentially, as when he is asleep, but sometimes we say someone sees and hears by the fact that he is doing the activity of seeing and hearing. Clearly, then, sensation and sensing may be spoken of in two ways, that is, in act or in potency.

356. Next, at to start with, then (417a14), he explains the above. For to speak of sensation as in act might seem contradictory to his previous statement that it was a certain being acted upon or being moved; for to be in act seems to pertain more to being active. So he explains that we are speaking of sensation in act as though we were saying that being acted upon or moved were a certain activity, that is, a certain state of being actual. For movement is a certain actuality, but an imperfect one (as he says in Physics 3), for it is the act of a being in potency, that is, of a changeable thing. (Aristotle, Physics 3.1, 201b31–32; cf. Aquinas, Commentary on Physics, bk. 3, lect. 2.) Just as motion is an act, then, being moved and sensing is a certain activity or being in act. The phrase, to start with, however, means that he will add something later to show how the senses become actual in fact.

357. Third, at all things are moved and acted upon (417a17), he shows how it follows from the above that the old position, that like senses like, cannot be true. All things that are in potency, he says, are acted upon and moved by an agent and something existing in act, namely, that which, when it makes actual things that are being acted upon, makes them like itself. In some sense, then, a thing is acted upon by both its like and its unlike (as we have already remarked). At first, and while it is being transformed and acted upon, there is dissimilarity; but at the end, when the thing has been transformed and acted upon, there is similarity. Therefore, so too, the sense, after it has been actually acted upon by the sensible object, is like it, but before, it is not like; the early philosophers went wrong because they missed this distinction.