On the Vital Principle/Book 3/Chapter 12
It is necessary to every living creature that it should have a nutritive principle in order that it may live and continue to live, from birth to death; for it is necessary to every thing generated that it should be capable of growth, development and decay, and as these cannot be carried on without nourishment, it is necessary to all reproductive and perishing beings that they should have a congenital nutritive function. But it is not necessary to all living beings that they should be sentient, nor can it be admitted that such as have simple or homogeneous bodies can have Touch, or that there can be an animal without Touch; neither can any beings be sentient but such as are receptive of form without the matter. It is necessary to an animal, however, that it should be sentient, if nature do nothing in vain—for all things in nature are for some end, or else they are accidents of things for some end; so that if there were any animal body fitted for progression without being sentient, it would perish, and could not attain to the end which is nature's design. How, in fact, is such a body to be nourished? As to creatures which are fixed, they obtain their nourishment on the spot where they have been produced. It is not possible then, that a body which is not fixed to one spot and which has been generated, should have living principle and judging faculties without being sentient. Nor can a creature spontaneously generated be sentient; Why, let us ask, should it be so? The sensibility is for the greater good either of the Vital Principle or the body; but neither of these can, in the case supposed, be effected by it, as the one will not through it think the better, nor the other be better fitted for its offices. Thus, there is no living body free to move which is not sentient. But if a body be sentient, it must necessarily be either homogeneous or compound—Now, homogeneous it cannot be, as in that case it would be without Touch, and Touch it must of necessity have. All which is proved thus—Since an animal is a living body, and all bodies are tangible, and tangible implies whatever is perceptible to Touch, it follows that the body of an animal must be sensible to Touch, if the animal is to preserve its existence. The other senses, as the sight, smell, hearing, perceive through other media; but if an animal when touching were without sensation, it could have no guide for avoiding some things or seizing others, and so circumstanced, it would not be possible for it to preserve its existence. The taste, therefore, is a kind of Touch, for taste is the sense for food and food is a tangible body; but sound, colour and odour neither nourish nor contribute to growth or decay. Thus, taste must of necessity be a kind of Touch from its being the sense which is perceptive both of what is tangible and nutritive; and, as these two senses are necessary to animals, it is manifest that there can be no animal without Touch.
The other senses, being for the higher good of animals, are allotted, not to all but, only to particular genera, as they are necessary to none but such as have the power of progression. If, indeed, such a creature is to preserve its existence, it must not only be sensible of objects when touching them, but be able also to perceive them when at a distance; and this can be effected if it be sensible through a medium, which, having been impressed and set in motion by the objects of perception reacts upon the percipient. And it is thus that the locomotive impulse acts until it cease in rest—that which impels something else communicates along with impulsion impelling power, and the motion is in a midspace; and as the first motor impels without having been impelled, so the last is impelled without impelling, and the intermediate links, of which there are several, both impel and are impelled. So is it too with respect to changes wrought in bodies, excepting that they are effected without change of locality—as if any one were to tinge a portion of wax, it would be in motion until it should be saturated; nothing like this, however, can happen to a stone, but it can to water, and that to a distance. The air is mobile in the highest degree, and, provided it be still and in one mass, it both acts and is acted upon. It is better, therefore, in the case of refraction, to assume that the air, in so far as it is one mass, (and it is so over every smooth surface,) is impressed by form and colour, rather than that visual rays issuing from the eye are refracted. Thus, the air, in the case of vision, gives motion to the sense, as if the impress upon wax had been transmitted to its extremity.
Notes[edit]
Note 1, p. 186. Simple or homogeneous bodies, &c.] There is no clue in Aristotle's writings to the meaning of "simple (or homogeneous) bodies" (τὸ σῶμα ἁπλοῦν), unless it be the Acalepha[1], "the body of which, like that of the oyster, is said to be altogether fleshy, but, unlike the oyster, to be without a shell; and it is further said to belong rather to plants than animals." But as in the following chapter it is shewn that an animal body, if homogeneous, cannot exist, so the tenor of the whole argument may be, to shew that no animal body can be homogeneous.
Note 2, p. 187. As to creatures which are fixed, &c.] These include such species of the Testacea as are fixed to one habitat, and derive nutriment from the water with which they are surrounded; but it is not easy to determine what is meant by the term ἀγεννήτον, (spontaneously generated,) as this mode of reproduction was attributed, by Aristotle, to some fishes and eels, which are certainly neither homogeneous nor insentient.
Note 3, p. 188. The other senses, being for, &c.] It may be questioned whether this description is absolutely true, as there are creatures which, although fixed to one habitat, not capable of progression that is, are endowed with all the senses, although it may be in some modified, or less active form than in the higher animals. For "the[2] nervous system has been detected in every division of the animal kingdom, and almost in every class, and it is everywhere connected with sensation and motion."
Note 4, p. 188. Be sensible through a medium, &c.] The medium, that is, made diaphanous and motive by colour or sound, acts, by a succession of undulations, upon the eye or the ear, and finally, through the humours of the former and the air in the latter, upon the sentient part of these organs; so that there is an evident analogy between these undulations and the impulses which maintain locomotion until lost in the state of rest. This succession of impulses may well apply to the changes which, without change of locality, are slowly and silently going on in bodies, and be compared to colouring matter which permeates and gradually combines with each molecule of wax up to saturation; but every substance cannot, of course, be thus affected—a stone, for instance, owing to the condensation of its particles, cannot be made receptive of colour.
Note 5, p. 189. The air is mobile in the highest degree, &c.] The nature and properties of the atmosphere were imperfectly known, as has been said, in the age of Aristotle. It was deemed necessary to sensation that the air should be still (for, when in motion, it was converted, according to opinion, into wind), and as one mass; and as this aggregation of the air could be only over smooth surfaces, the outer coat of the eye (the cornea) seemed, by its smoothness, to favour Aristotle's doctrine, that vision is through a medium, and completed, by refraction, at the bottom of the organ. The medium, set in motion, by colour, was said to give motion, by successive impulses, to the air over the cornea, which communicated the impulse to the organ within; and this superseded the doctrine that vision is produced by rays emanating from the eye. Thus, "the air (that over the cornea), in its turn, sets vision in motion;" but the last clause of the sentence is very obscure, and offers, as some commentators have said, "great difficulties." It may, perchance, be a continuation of the analogy and suggest that, as colouring matter acts, successively, until each particle is saturated, so the impulse is transmitted to the cornea, and finally, from it, to the visual faculty within.