Lecture 23 — Touch: its medium, object, and organ

A problem arises on the assumption that every body has depth, that is, the third dimension. Bodies having between them a medium which is a body cannot directly touch one another. Now, every liquid involves body, and so does everything moistened; it must either be water or contain water. But things that touch one another in water must necessarily have water as a medium covering their extremities, unless these last be dry. If this is true, it is impossible for one body to touch another in water. The same holds good of air (for air is to the things that are in it as water to things in water), although this fact is less evident to us, just as animals that live in water are unaware that bodies that touch in water are all wet.

The question, then, is whether there is one way of sensing for all objects of sense, or different ways for diverse objects. The latter at first sight seems to be the case—taste and touch being effected by contact, the others from a distance.

But this is not so: we do sense the hard and the soft through something else, just as we do the audible and the visible and the odorous. But of these objects, some operate at a distance, others close at hand. That is why the fact escapes us: we do sense everything through a medium, but the fact is not evident in the latter cases.

Indeed, as we said before, if we were to sense all tangible objects through a membrane, not knowing what was interpolated, we should think we touched the objects themselves, as we now do in air and water: for in these cases we think we touch the objects and that there is no medium.

But the tangible differs from the visible and the audible; for we sense the latter in that the medium itself moves something in us; whereas the tangible does not affect us through the medium so much as with the medium, simultaneously, as when one is struck on a shield. For the shield does not strike its holder after it is itself struck; but the two are struck at once.

It would seem in general that flesh and the tongue stand to the sense organ precisely as water and air to sight and hearing and smell, each to its respective sense. When the sense organ is touched (as when one places a white object on the surface of the eye), no sensation is produced in either case.

Hence, the organ of the tangible is internal, for the same thing happens in this sense as in the others. For what is placed on the organ they do not sense; what, however, is placed on flesh, they do sense. Flesh, then, is the medium of touch.

Tangible objects vary, therefore, with differences of body as such—I mean the differences by which the elements are distinguished, as hot and cold, wet and dry, as has been stated earlier in our work on the elements.

The sense organ for these, the tactile, in which the sense called touch primarily resides, is the part in potency to these qualities. For to sense is a certain being acted upon. Hence, whatever makes the organ to be such as it itself is actually, does so with the organ being in potency thereto. Hence, we do not sense what has heat or cold or hardness or softness to an exact likeness of our own heat, and so forth, but rather the extremes of these: the sense being, as it were, in a mean state between the contrary extremes in the sensible objects, which is how it discerns them.

For a mean is discriminative; in the presence of either extreme, it becomes the contrary one. Hence, as whatever is to sense black or white must have neither of these in itself actually, but both potentially (and so with the other sense-objects), so touch must be actually neither hot nor cold.

Further, as sight is, in a certain way, of the visible and the invisible (and similarly with the rest of such opposites), so touch is of both the tangible and the intangible. The intangible is that which has the distinguishing quality of tangibles to a very small extent, as air is affected, and also the excessively tangible, such as things destructive.

We have now said in outline something about each of the senses.

530. A problem arises on the assumption (423a21). Having shown how the sense of touch requires a medium that is of the nature of the one who touches, the Philosopher goes on to ask whether it needs an extraneous medium. And his answer contains two points.

First, he shows that touch does not occur without an extraneous medium.

Second, at the question, then, is whether (423b1; [541]), he shows how touch and taste differ from other senses, which sense through an extraneous medium.

First, then, he remarks that in the case of touch, one might doubt whether it has an extraneous medium, since we have seen that the medium of touch is a natural part of us. (Lect. 22, n. 527.) Every body has depth, that is, a third dimension. And indeed it is clear that every body has three dimensions: length, breadth, and depth; whence it follows that whenever two bodies have a medium between them, the former bodies are not in direct contact; they must be divided by some dimension that is between them.

531. It is also evident that wherever there is a liquid or moistened medium, there must be a body of some sort. For wetness is a quality and must exist in a body as its subject. Either then it resides in a body essentially, and then there is a liquid, for example, water; or it exists in virtue of some adventitious body, in which case there is something that is wet, that is, having water either on its surface only or on its surface and throughout it as well; so he says that every humid or wet body must either be water or contain water.

Now it is clear that bodies which touch one another in water have water as a medium between them covering their extremities—unless it should be said that they are dry in water, which is impossible; for whatever things are in water must be wet, in the sense that water covers their surfaces. Water, then, is the medium between two things touching each other in water; whence it follows that it is impossible for one body to touch another in water immediately. And likewise, the same holds good of air, which is naturally a fluid, as water is.

532. For the air is to the things that are in it as water to things in water; but it is less evident to us that air is a medium of this kind than that water is, on account of our being continually in the air, so that it envelops us imperceptibly. In the same way, animals that live in water are not aware that when two bodies touch one another, their surfaces are wet; for, being habitually in water, they do not notice the water between them and the bodies they are touching. Another reason why the fact is less evident to us in the case of air than in water is that air is more subtle and less perceptible to sense. Whenever, then, we touch anything, there is always a medium between ourselves and the thing touched, whether air or water.

533. But a question suggests itself here. For the medium for any sense should be lacking in the sensible qualities perceived by it—for example, the transparent has no color. But obviously air and water have tangible qualities; so it seems they cannot be the medium for the sense of touch.

534. Averroes answers that we are not acted upon by pure air or pure water. (Averroes, In De anima, bk. 2, 115, v. 43–81.) For nothing is acted upon, he says, save by its contrary, in accordance with the principle of passivity; but air and water are not contrary to us—they are indeed akin to us, for they are related to us as a place is related to what is contained by it. Hence our touch is not acted upon by air and water, but by qualities extraneous to them. For what tangible qualities are sensed in air and water are due to their combination with extraneous bodies, as he says, for as fire never loses its heat, so water never loses its coolness; and if we ever sense it as hot, this is due to the admixture of something extraneous.

535. Now, this reply contains several errors.

First, it is false to assert that air and water do not act on our bodies because we are akin to these as what is in place to the place. For clearly our bodies get their natural place as they do their natural movement, from the element that predominates in them; and, consequently, our bodies are related to place and to the containing bodies as the elements themselves are related to place. But the elements mutually alter one another at their extremities, as Aristotle shows in Meteorology 1. (Aristotle, Meteorology 1.3, 340b4–10, 19–20; cf. Aquinas, Commentary on Meteorology, bk. 1, lect. 3.) Our bodies, therefore, are naturally subject to the influence of the elements.

536. Again, everything in potency is naturally apt to be acted upon by what is in act; and our bodies, being in a mean state between the extremes of the tangible qualities, which are in the elements, are related to these elementary qualities as potency to act. For, as we shall see, a mean is in potency to the extremes. It is evident, then, that our bodies are naturally apt to be modified by the elemental qualities and to sense them.

537. This error of Averroes comes from his failing to distinguish between the elements as contrary to each other and as akin to and containing one another (as place contains what is in a place).

538. Note, then, that the elements can be considered in two ways: in one way, in terms of active and passive qualities, and in this sense they are contrary to one another and act upon one another at their points of contact; but in another way, in terms of their substantial forms, which arise from the influence of the heavenly body. Whence, the elements have more of form to the degree that they are closer to the heavenly body; and since it pertains to form to contain, and it relates to things in the account of a whole, it follows that the higher body contains the lower and stands to the latter as a whole to a distinct part, which is the relation of a place to what is placed. This is also why the function of placing and containing is derived to the elements from the primary container, the heavenly body. Whence place and local motion are not attributed to the elements in virtue of their active and passive qualities, but rather they follow the elements’ substantial forms.

539. Another error is that he says that air and water are not altered save by mixing with something extraneous. For it is evident that air and water are in some degree destructible; but destruction and generation can take place in the elements without any mixing and yet these follow after alteration, as is proved in On Generation. (Aristotle, On Generation and Corruption 1.2, 315b20–24; 317a31; cf. Aquinas, Commentary on Aristotle’s On Generation and Corruption, bk. 1, lect. 2, 3, 5, 10; bk. 2, lect. 8.) Therefore, so long as water remains in the form of water, its natural quality can be changed without the admixture of anything extraneous. The case of fire is quite different; being the most formal and active of the elements, the rest are material in relation to it, as Aristotle says in Meteorology 4. (Aristotle, Meteorology 4.1, 379a16; cf. Aquinas, Commentary on Meteorology, bk. 4, lect. 2, 3, 15.)

540. It may be said, then, that air and water are easily changeable by extraneous qualities, especially in small quantities, such as the amount of air and water between two bodies that touch one another. This is why the sense of touch is not impeded by the medium of water or air. And, of the two, air is less an impediment than water, for its tangible qualities are scarcely perceptible at all. But an intensification of the tangible qualities of air or water, as when they become extremely hot or cold, does of course impede the sense of touch.

541. Then, when he says, the question (423b1), he shows the difference between touch and taste, on the one hand, and the rest of the senses on the other.

First, he rejects a supposed ground of differentiation.

Second, he states the true difference, at but the tangible differs (423b12; [543]).

In the first place, then, he says that this question of the extraneous medium of touch leads one to ask whether the sensation of all sense-objects occurs in one way or diversely for diverse objects—it seems, at first sight, that touch and taste sense by immediate contact, while the other senses apprehend their objects from a distance.

542. But this difference is illusory; we do in fact sense the hard and the soft and other tangible qualities through something else, an extraneous medium, just as the objects of other senses, the audible, the visible, and the odorous. But while the latter objects are sometimes at a distance from the sense, the tasteable and tangible are so close that the medium is almost imperceptible and therefore escapes us. We do sense every sense-object through an extraneous medium, but it is not evident in taste and touch; as we said before, if we were to sense all tangible objects through a membrane, not knowing what was interpolated, we should feel through a medium in a manner as we do now when we actually feel in air or water. (Lect. 22, n. 526.) For as it is, we think we touch the sense-objects themselves and that there is no medium.

543. Then, at but the tangible differs (423b12), having rejected the false difference, he states the true one, observing that tangible objects differ from the visible and the audible in that, whereas we sense the latter because they set in motion the medium, and the medium in turn moves us, we sense tangible objects, not as moved by an extraneous medium, but as being moved simultaneously with the medium, by the sense-object. It is as when a man is struck on a shield; the shield, after it is itself struck, does not then strike the man, but man and shield are struck at once.

544. Nor is this at once to be understood in the order of time only; for in sight the medium is affected by the visible and the eye by the medium, and yet sight occurs without succession in time. Smelling and hearing, however, take place with some temporal succession, as it is said in On Sense and What is Sensed. (Aristotle, On Sense and What is Sensed, ch. 1; ch. 5; ch. 6, 446b25–28, 447a8–11; cf. Aquinas, Commentary on Aristotle’s On Sense and What is Sensed, cc. 2, 14.) The succession is due to the way the cause of the action operates; for whereas in the other senses a change in the medium is itself the cause of the sense being affected, it is not so in touch; for in other sensations the medium is present of necessity, while it is only, as it were, an accidental accompaniment of touch, due to the fact, for example, that the bodies in contact are moist.

545. Next, when he says, it would seem in general (423b17), he concludes to the truth about the sense of touch:

first, with regard to the medium;

second, with regard to the organ, at tangible objects vary (423b26; [546]);

third, with regard to the object apprehended by this sense, at further, as sight (424a10; [549]).

First, then, he observes that the flesh and the tongue seem to be related to the organ of touch as water and air to the organs of sight, hearing and smell. Now, in none of these latter senses can sensation occur if the organ itself is touched; thus, a white object placed on the surface of the eye is invisible. Thus it is clear that the organ of touch is internal, for this sense works in the same way as in others; and since animals do not sense sense-objects apart from their sense organs, if animals can sense sense-objects placed on their flesh, it is evident that flesh is not precisely a sense organ but rather a medium of sensation.

546. Then, at tangible objects (423b26), he explains the nature of the organ of touch. Tangible qualities, he says, are the differences of body precisely as body, that is, those differences which diversify the elements from each other, namely, dry and wet, hot and cold, of which Aristotle has spoken in his work on the elements, On Generation and Corruption. (Aristotle, On Generation and Corruption 2.2–3, 329b7–331a6; cf. Aquinas, Commentary on Aristotle’s On Generation and Corruption, bk. 2, lect. 2–3.)

547. For it is clear that the organ for the tactile, in which the sense called touch primarily resides, is the part of the body in potency to these qualities. For every sense organ is acted upon by its sense-object, because to sense is a certain being acted upon. Hence, the sense-object, which is the agent, makes it be actually in a condition similar to itself, for it was in potency to this. This is why, in the degree that the organ of touch already actually has any quality, it does not sense this quality. For we do not sense a thing as hot or cold, hard or soft, insofar as these qualities are already present in the organ of touch; rather, we sense the extremes of the tangible qualities, as though the organ of touch has been established in a certain mean state between contrasted tangible qualities. This is why the organ of touch discerns between the extremes of the tangibles. For a mean is discriminative of its extremes, because it can be acted upon by each of the extremes insofar as it is related to one of them in the account of the other: for example, as compared with heat the tepid is cool, but as compared with cold, it is warm. Thus the mean is acted upon by both extremes, being in a way the contrary of each. As the organ that is to sense white and black must have neither of these actually but both of them potentially (and the like is true of the other senses), so also is it, and necessarily, in the sense of touch: its organ is neither hot nor cold, but in potency to both.

548. But this occurs in a special way in touch. In sight, for instance, the organ in potency to black and white is quite free from the entire genus of black and white, for it is wholly lacking in color. But in the case of touch, the organ cannot be completely deprived of the genus of hot and cold, moist and dry, for it is composed of elements having these qualities essentially. Rather, the organ of touch is in potency to its objects to the degree that it is a mean between the contraries, for a mean is in potency to either extreme. Whence it follows that the closer an animal’s composition approaches the mean state, the finer will be its sense of touch; and that is why man, of all animals, has the finest touch, as we have seen. (Lect. 19, nn. 482–83.)

549. Next, at further, as sight (424a10), he concludes about the object of touch. As sight, he says, is of the visible and invisible, and the other senses are also of opposites (as hearing of sound and silence), so touch is of both the tangible and the intangible. Intangible is said in two ways: either of that which has a tangible quality to an excess such that it destroys the sense, like fire; or of that which has very slight tangibility, like air. Both are called intangible because both are hard to perceive by touch.

550. Summing up, he says that he has spoken in outline (424a15), that is, in a summary manner, of each of the several senses. For he deals with them more in detail in On Sense and What is Sensed. (Aristotle, On Sense and What is Sensed, cc. 1ff.; cf. Aquinas, Commentary on Aristotle’s On Sense and What is Sensed, cc. 2ff.)