De Anima

Every living thing, then, must have the nutritive soul and in fact has a soul from its birth till its death. For what has been born must necessarily grow, reach maturity and decline, and for these processes nutriment is indispensable. It follows, then, of necessity that the nutritive faculty is present in all things that grow and decay. But sensation is not necessarily present in all living things. For wherever the body is uncompounded there can be no sense of touch [yet without this sense animal existence is impossible]: nor, again, in those living things which are incapable of receiving forms apart from matter. But the animal must of necessity possess sensation, if nature makes nothing in vain: for everything in nature subserves an end or else will be an accessory of things which subserve an end. Now every living body having the power of progression and yet lacking sensation would be destroyed and never reach full development, which is its natural function. For how in such a case is it to obtain nutriment? Motionless animals, it is true, have for nutriment that from which they have been developed. But a body, not stationary, but produced by generation, cannot possibly have a soul and an intelligence capable of judging without also having sensation. [Neither can it, if it be not generated.] For why should it have the one without the other? Presumably for the advantage either of the soul or of the body. But neither of these alternatives is, in fact, admissible. For the soul will be no better able to think, and the body will be no better off, for the absence of sensation. We conclude, then, that no body that is not stationary has soul without having sensation.

But, further, the body, assuming that it has sensation, must be either simple or composite. But it cannot be simple, for then it would not have touch, and this sense is indispensable. This is clear from the following considerations. The animal is an animate body. Now body is always tangible and it is that which is perceptible by touch which is tangible: from which it follows that the body of the animal must have tactile sensation, if the animal is to survive. For the other senses, that is to say, smell, sight, hearing, have media of sensation, but a being which has no sensation will be unable when it comes into contact with things to avoid some and seize others. And if this is so, it will be impossible for the animal to survive. This is why taste is a kind of touch, for taste is of nutriment and nutriment is body which is tangible; whereas sound, colour and smell afford no nourishment and promote neither growth nor decay. So that taste also must be a kind of touch, because it is a sensation of that which is tangible and nutritive. These two senses, then, are necessary to the animal, and it is plain that without touch no animal can exist.

But the other senses are means to well-being, and are necessary, not to any and every species of animal, but only to certain species, as, for example, those capable of locomotion. For, if the animal capable of locomotion is to survive, it must have sensation, not only when in contact with anything, but also at a distance from it. And this will be secured if it can perceive through a medium, the medium being capable of being acted upon and set in motion by the sensible object, and the animal itself by the medium. Now that which causes motion from place to place produces a change operating within certain limits, and that which propels causes the thing propelled to propel in turn, the movement being transmitted through something intermediate. The first in the series initiates motion and propels without being itself propelled, while the last is simply propelled without propelling; the numerous middle terms of the series both propel and are propelled. So it is also with qualitative change, except that what is subject to this change remains in the same place. Suppose we were to dip something into wax, the movement in the wax would extend just so far down as we had dipped the object, whereas in the like case a stone is not moved at all, while water is disturbed to a great distance and air is disturbed to the farthest extent possible and acts and is acted upon as long as it remains unbroken. And, to revert to the reflection of light, that is why, instead of holding that the visual ray leaving the eye is reflected, it would be better to say that the air is acted upon by the shape and colour, so long as it is one and unbroken. This is the case over any smooth surface: and accordingly the air acts on the organ of sight in turn, just as if the impress on the wax had penetrated right through to the other side.