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.