On the Vital Principle/Book 3/Prelude to Chapter 9
This and the two following chapters are upon the parts or powers rather, which give to animals locomotion; but, as the nervous even the muscular system had not then been made out, the text is encumbered, occasionally, as might be expected, with speculations which may now seem idle, and distinctions which are almost futile. Aristotle[1] makes “animals to move and be moved for the sake of something, which is the limit of all their movements; and the moving powers of an animal are, perceptibly, he adds, thought and imagination, election, will and desire, which are all referrible to mind and appetite, εἰς νοῦν καὶ ὄρεξιν. Thus, as imagination and perception are alike able to direct an animal, they are in one and the same relation to the mind. The argument, in fact, dwells upon the motive as well as the object for progression, without a word concerning the agency by which it is to be effected, as if the muscular power of the body were unknown, or regarded only as the seat or source of the touch; and yet the flesh was said to be the origin[2] and very body of an animal. The strength[3] of all animals is, he adds, in the tendons (ἡ ἰσχὺς ἐν τοῖς νεύροις), and, therefore, strength is greatest when they are full grown; for the young have weak joints and deficient sinews.
Since the Vital Principle of animals has been defined by the two faculties of judgment (which is the office of thought with sentient perception), and of locomotion, let us now, having dwelt sufficiently upon sensation and mind, proceed to consider, with respect to the motor power, what part of the Vital Principle it may be. Let us consider, that is, whether it is a part of Vital Principle and separate from it, substantively or abstractedly, or whether it is Vital Principle as a whole; and if it be a part, whether it is something peculiar and exclusive of those usually attributed to Vital Principle, and which have been alluded to, or whether it is to be considered as one of them.
But a difficulty at once presents itself, both in