The existent universe is a scale of being lying between the two extremes of formless matter and matterless form. But this must not be merely asserted, as a general [{289}] principle. It must be carried out in detail. The passage of matter into form must be shown in its various stages in the world of nature. To do this is the object of Aristotle's Physics, or philosophy of nature.
If nature is to be understood, we must keep in mind certain general points of view. In the first place, since form includes end, the entire world-process, as passage of matter into form, is essentially movement towards ends. Everything in nature has its end and function. Nothing is purposeless. Nature seeks everywhere to attain the best possible. Everywhere we find evidences of design and of rational plan. Aristotle's philosophy of nature is essentially teleological. This does not, however, exclude the principle of mechanism, and to investigate mechanical causes is part of the duty of science. But mechanical causes turn out in the end to be teleological, because the true efficient cause is the final cause.
But if nothing in nature is aimless or useless, this is not to be interpreted in a narrow anthropocentric spirit. It does not mean that everything exists for the use of man, that the sun was created to give him light by day, the moon by night, and that plants and animals exist only for his food. It is true that, in a certain sense, everything else sublunary is for man. For man is the highest in the scale of beings in this terrestrial sphere, and therefore as the higher end, he includes all lower ends. But this does not exclude the fact that lower beings have each its own end. They exist for themselves and not for us.
Another mistake which we must avoid is to suppose that the design in nature means that nature is conscious of her designs, or, on the other hand, that there is any [{290}] existent consciousness outside the world which governs and controls it. The latter supposition is excluded by the fact that God is not an existent conscious person, the former by its own inherent absurdity. The only being upon this earth who is conscious of his ends is man. Such animals as bees and ants appear to work rationally, and their activities are clearly governed by design. But it is not to be supposed that they are reasoning beings. They attain their ends instinctively. And when we come to inorganic matter, we find that even here its movements are purposive, but no one could suppose them deliberate and conscious. These manifold activities of lower nature are indeed the work of reason, but not of an existent or self-conscious reason. And this means that instinct, and even mechanical forces such as gravitation are, in their essence, reason. It is not that they are created by reason, but that they are reason, exhibiting itself in lower forms. In commenting upon Plato's dualism of sense and reason, I remarked that any true philosophy, though recognizing the distinction between sense and reason, must yet find room for their identity, and must show that sense is but a lower form of reason. This idea Aristotle thoroughly understood, and sought to show, not merely that sense is reason, but even that the activities of inorganic matter, such as gravitation, are so. In the result, nature, though working through reason, is not conscious of the fact, does so blindly and instinctively, and is compared to a creative artist, who forms beautiful objects by instinct, or, as we should say, by inspiration, without setting before his mind the end to be attained or the rules to be observed in order to attain it.
In the process of nature, it is always form which impels, matter which retards and obstructs. The entire world-movement is the effort of form to mould matter, but, just because matter has in itself a power of resistance, this effort does not always succeed. This is the reason why form cannot exist without matter, because it can never wholly overcome the clogging activity of matter, and therefore matter can never be wholly moulded into form. And this explains, too, the occasional occurrence in nature of freaks, monstrosities, abortions, and unnatural births. In these the form has failed to mould the matter. Nature has failed to attain her ends. Science, therefore, should study the normal and natural rather than the abnormal and monstrous. For it is in the normal that the ends of nature are to be seen, and through them alone nature can be understood. Aristotle is fond of using the words "natural" and "unnatural," but he uses them always with this special meaning. That is natural which attains its end, that in which the form successfully masters the matter.
No doctrine of physics can ignore the fundamental notions of motion, space, and time. Aristotle, therefore, finds it necessary to consider these. Motion is the passage of matter into form, and it is of four kinds. The first is motion which affects the substance of a thing, origination and decease. Secondly, change of quality. Thirdly, change of quantity, increase and decrease. Fourthly, locomotion, change of place. Of these, the last is the most fundamental and important.
Aristotle rejects the definition of space as the void. Empty space is an impossibility. Hence, too, he disagrees with the view of Plato and the Pythagoreans that [{292}] the elements are composed of geometrical figures. And connected with this is his repudiation of the mechanical hypothesis that all quality is founded upon quantity, or upon composition and decomposition. Quality has a real existence of its own. He rejects, also, the view that space is a physical thing. If this were true, there would be two bodies occupying the same place at the same time, namely the object and the space it fills. Hence there is nothing for it but to conceive space as limit. Space is, therefore, defined as the limit of the surrounding body towards what is surrounded. As we shall see later, in another connexion, Aristotle did not regard space as infinite.
Time is defined as the measure of motion in regard to what is earlier and later. It thus depends for its existence upon motion. If there were no change in the universe, there would be no time. And since it is the measuring or counting of motion, it also depends for its existence upon a counting mind. If there were no mind to count, there could be no time. This presents difficulties to us, if we conceive that there was a time when conscious beings did not exist. But this difficulty is non-existent for Aristotle, who believed that men and animals have existed from all eternity. The essentials of time, therefore, are two: change and consciousness. Time is the succession of thoughts. If we object that the definition is bad because succession already involves time, there is doubtless no answer possible.
As to the infinite divisibility of space and time, and the riddles proposed thereupon by Zeno, Aristotle is of opinion that space and time are potentially divisible [{293}] ad infinitum, but are not actually so divided. There is nothing to prevent us from going on for ever with the process of division, but space and time are not given in experience as infinitely divided.