History of Western Philosophy - Bertrand Russell [121]
In relation to physics, Aristotle's imaginative background was very different from that of a modern student. Nowadays, a boy begins with mechanics, which, by its very name, suggests machines. He is accustomed to motor-cars and aeroplanes; he does not, even in the dimmest recesses of his subconscious imagination, think that a motor-car contains some sort of horse in its inside, or that an aeroplane flies because its wings are those of a bird possessing magical powers. Animals have lost their importance in our imaginative pictures of the world, in which man stands comparatively alone as master of a mainly lifeless and largely subservient material environment.
To the Greek, attempting to give a scientific account of motion, the purely mechanical view hardly suggested itself, except in the case of a few men of genius such as Democritus and Archimedes. Two sets of phenomena seemed important: the movements of animals, and the movements of the heavenly bodies. To the modern man of science, the body of an animal is a very elaborate machine, with an enormously complex physico-chemical structure; every new discovery consists in diminishing the apparent gulf between animals and machines. To the Greek, it seemed more natural to assimilate apparently lifeless motions to those of animals. A child still distinguishes live animals from other things by the fact that they can move of themselves; to many Greeks, and especially to Aristotle, this peculiarity suggested itself as the basis of a general theory of physics.
But how about the heavenly bodies? They differ from animals by the regularity of their movements, but this may be only due to their superior perfection. Every Greek philosopher, whatever he may have come to think in adult life, had been taught in childhood to regard the sun and moon as gods; Anaxagoras was prosecuted for impiety because he thought that they were not alive. It was natural that a philosopher who could no longer regard the heavenly bodies themselves as divine should think of them as moved by the will of a Divine Being who had a Hellenic love of order and geometrical simplicity. Thus the ultimate source of all movement is Will: on earth the capricious Will of human beings and animals, but in heaven the unchanging Will of the Supreme Artificer.
I do not suggest that this applies to every detail of what Aristotle has to say. What I do suggest is that it gives his imaginative background, and represents the sort of thing which, in embarking on his investigations, he would expect to find true.
After these preliminaries, let us examine what it is that he actually says.
Physics, in Aristotle, is the science of what the Greeks called 'phusis' (or 'physis'), a word which is translated 'nature', but has not exactly the meaning which we attach to that word. We still speak of 'natural science' and 'natural history', but 'nature' by itself, though it is a very ambiguous word, seldom means just what 'phusis' meant. 'Phusis' had to do with growth; one might say it is the 'nature of an acorn to grow into an oak, and in that case one would be using the word in the Aristotelian sense. The 'nature' of a thing, Aristotle says, is its end, that for the sake of which it exists. Thus the word has a teleological implication. Some things exist by nature, some from other causes. Animals, plants, and simple bodies (elements) exist by nature; they have an internal principle of motion (the word translated 'motion' or 'movement' has a wider meaning than 'locomotion'; in addition to locomotion it includes change of quality or of size). Nature is a source of being moved or at rest. Things 'have a nature' if they have an internal principle of this kind. The phrase 'according to nature' applies to these things and their essential attributes. (It was through this point of view that 'unnatural' came to express