History of Western Philosophy - Bertrand Russell [122]
Nature belongs to the class of causes which operate for the sake of something. This leads to a discussion of the view that nature works of necessity, without purpose, in connection with which Aristotle discusses the survival of the fittest, in the form taught by Empedocles. This cannot be right, he says, because things happen in fixed ways, and when a series has a completion, all preceding steps are for its sake. Those things are 'natural' which 'by a continuous movement, originated from an internal principle, arrive at some completion' (199b).
This whole conception of 'nature', though it might well seem admirably suited to explain the growth of animals and plants, became, in the event, a great obstacle to the progress of science, and a source of much that was bad in ethics. In the latter respect, it is still harmful.
Motion, we are told, is the fulfilling of what exists potentially. This view, apart from other defects, is incompatible with the relativity of locomotion. When A moves relatively to B, B moves relatively to A, and there is no sense in saying that one of the two is in motion while the other is at rest. When a dog seizes a bone, it seems to common sense that the dog moves while the bone remains at rest (until seized), and that the motion has a purpose, namely to fulfil the dog's 'nature'. But it has turned out that this point of view cannot be applied to dead matter, and that, for the purposes of scientific physics, no conception of an 'end' is useful, nor can any motion, in scientific strictness, be treated as other than relative.
Aristotle rejects the void, as maintained by Leucippus and Democritus. He then passes on to a rather curious discussion of time. It might, he says, be maintained that time does not exist, since it is composed of past and future, of which one no longer exists while the other does not yet exist. This view, however, he rejects. Time, he says, is motion that admits of numeration. (It is not clear why he thinks numeration essential.) We may fairly ask, he continues, whether time could exist without the soul, since there cannot be anything to count unless there is someone to count, and time involves numeration. It seems that he thinks of time as so many hours or days or years. Some things, he adds, are eternal, in the sense of not being in time; presumably he is thinking of such things as numbers.
There always has been motion, and there always will be; for there cannot be time without motion, and all are agreed that time is uncreated, except Plato. On this point, Christian followers of Aristotle were obliged to dissent from him, since the Bible tells us that the universe had a beginning.
The Physics ends with the argument for an unmoved mover, which we considered in connection with the Metaphysics. There is one unmoved mover, which directly causes a circular motion. Circular motion is the primary kind, and the only kind which can be continuous and infinite. The first mover has no parts or magnitude and is at the circumference of the world.
Having reached this conclusion, we pass on to the heavens.
The treatise On the Heavens sets forth a pleasant and simple theory. Things below the moon are subject to generation and decay; from the moon upwards, everything is ungenerated and indestructible. The earth, which is spherical, is at the centre of the universe. In the sublunary sphere, everything is composed of the four elements, earth, water, air, and fire; but there is a fifth element, of which the heavenly bodies are composed. The natural movement of the terrestrial elements is rectilinear, but that of the fifth element is circular. The heavens are perfectly spherical, and the upper regions are more divine than the lower. The stars and planets are not composed of fire, but of the fifth element; their motion