Whether touch is in the flesh or in something analogous to it.
Now one asks about the members serving the senses. And first, whether touch is in the flesh or in something analogous to it.
1. It seems so. Because touch is in any given part of the body. But nothing is more abundant in any given part than is flesh or something analogous to it. Therefore, etc.
2. Again, touch is either in the flesh or in nerve or bone. But there are many animals that do not have nerves and bones, in whom, nevertheless, touch is present. Therefore, etc.
3. Again, touch consists in the mean proportion and balance of tangibles; but this is found particularly in flesh. Therefore, etc.
The Philosopher demonstrates the opposite in book two of On the Soul, because "a sensible placed on a sense is not perceived," but a tangible, like heat, placed on the flesh is perceived. Therefore, flesh is not the organ of touch.
To the question one must say that touch can exist in something in four ways. It may do so just as it does in an organ and in a principle, and thus "touch is in something that is like the heart," as is said in On Sense and the Sensed. In another way, touch is in something which makes the operation of touch manifest, and thus touch is in the brain. In the third way it is in something that transmits the tactile power, and in this way it is in the nerves. In the fourth way it is in something as if in a medium, and in this way it is in the flesh. Thus, when the Philosopher says that "touch is in the flesh," he does not mean that it is in the flesh as in an organ, unless the term flesh is extended to blood and nerve and cartilage and such things.
1. On to the arguments. To the first, one must reply that touch can be considered in two ways: either with respect to substance or with respect to operation. With respect to substance, it is in every part of the body, because sensation is in every part of the body simply as touch and touch alone. If it is considered with respect to operation, then it is in a determinate part, for example, in the nerves or in something similar to nerves. Thus the Philosopher says concerning sight in On Sense and the Sensed that "sight is not in the outer surface of the eye, but in something interior. "
2. To the second, one must reply that although nerve and bone properly understood are not in every animal, nevertheless each one has something analogous to them, just as, in the same way, not each one has flesh but each one has something analogous to flesh.
3. To the third, one must reply that some nerves are more earthy and for that reason insensitive, but others are better proportioned, like the nerves of the muscles. Now muscle is just like nerve-filled flesh, and such nerves are especially sensitive. So an injury to them is especially painful. Therefore, some nerves are reduced more to balance and to the mean than is flesh as properly understood, unless the term "flesh" is extended to other things, as the nerves of the muscles are more tempered, etc.
Whether sensation exists in uniform members [membris similibus] and whether operation exists in non-uniform members.
One asks whether sensation exists in uniform members and whether operation exists in non-uniform members.
1. It seems not. For in a living body two things are present: distance from its opposite and organization. By its distance from its opposite it is disposed to life; and by organization it is disposed to operation. But sensation is an operative power. Therefore, sensation exists in an organic part.
2. Again, the animal power is divided into a motive and a sensitive power. But the motive power exists in the organic parts. Therefore, so too does the sensitive power.
The same seems to be true with respect to vision, taste, and hearing. For they exist in organic parts and are thus present in the non-uniform parts.
The Philosopher says the opposite.
One must say that sensation exists in uniform parts, and the operative power in non-uniform parts. The reason for this is that sense is a passive power and so it is related to matter. But the operative (that is, active) powers are related to form. Therefore, since whatever is received is related to the one able to receive it, the material parts will correspond to the sensitive power and the formal parts to the operative power. But uniform parts are material when compared with the non-uniform. And for this reason, etc.
Nevertheless, one must understand that, of all the senses, touch especially operates through a uniform part. The reason for this is twofold: one stemming from the sense of touch itself, and the other from the object. The reason stemming from the sense itself is that it is especially diffused throughout the body. For it is just like a guardian for the machine that is the animal body, because it guards it by guarding it from heat, cold, and dryness, and so too from other things. And thus it is reasonable that it should be in a part that can be more extended throughout the body. And a homogenous partlike nerve, or flesh, or something analogous to themis one such as this. The reason stemming from the object appears to be much the same case, because touch discerns the extremes [ excellentias] of tangible objects. Therefore, it is necessary that it exist in particular balance. But greater balance is observed in uniform than in nonuniform parts.
1. On to the arguments. To the first, one must reply that act is double: there is a first and second act. The first act is like life, sense, and other such things. And a part corresponds to an act to the extent that it is distant from its opposite. The second act is something like to live, to sense, and the like. And this too is double: one is proximate, and the other is remote. The proximate is something like to live and to touch, without which there is no life. The more remote is to hear, to see, to smell, and the like. And this is why to live and to touch, owing to their proximity to a first act, can exist in uniform parts, better than to see or to hear, which are in organic parts.
2. To the second, one must reply that although the animal power may be divided into motive and sensitive, nevertheless the motive is active and the sensitive is passive, and this is why the explanation [ ratio] is not the same for each.
3. To the third, the cause has already been given why touch can exist better in uniform parts than can hearing. This is owing to its proximity to the first act. Thus, the first sense is the foundation for the others, and the organs for the others are founded on the organ for touch, which would not be so were touch not based in something uniform.
-  As Ar. notes at De anima 424a5f., each of the senses has its peculiar object, and the object has its own qualities. The object of touch (the "tangible") is, for example, hot or cold. But touch itself is neither hot nor cold, although it