Thus the fundamental fact everywhere present in Nature is "change," "process," "motion." Since motion in the literal sense of change of position is involved as a condition of every such process, and such motion requires space through which to move and time to move in, the doctrine of space and time will also form part of Physics. Hence a great part of Aristotle’s special lectures on Physics is occupied with discussion of the nature of space and time, and of the continuity which we must ascribe to them if the "continuous motion" on which the unbroken life of the universe depends is to be real Aristotle knows nothing of the modern questions whether space and time are "real" or only "phenomenal," whether they are "objective" or "subjective." Just as he simply assumes that bodies are things that really exist, whether we happen to perceive them or not, so he assumes that the space and time in which they move are real features of a world that does not depend for its existence on our perceiving it. His treatment of space is singularly _naïf_. He conceives it as a sort of vessel, into which you can pour different liquids. Just as the same pot may hold first wine and then water, so, if you can say, "there was water here, but now there is air here," this implies the existence of a receptacle which once held the water, but now holds the air. Hence a jug or pot may be called a "place that can be carried about," and space or place may be called "an immovable vessel." Hence the "place" of a thing may be defined as the boundary, or inner surface, of the body which immediately surrounds the thing. It follows from this that there can be no empty space. In the last resort, "absolute space" is the actual surface of the outermost "heaven" which contains everything else in itself but is not contained in any remoter body. Thus all things whatever are "in" this "heaven." But it is not itself "in" anything else. In accord with the standing Greek identification of determinate character with limitation, Aristotle holds that this outermost heaven must be at a limited distance from us. Actual space is thus finite in the sense that the volume of the universe could be expressed as a finite number of cubic miles or yards, though, since it must be "continuous," it is infinitely divisible. However often you subdivide a length, an area, or a volume, you will always be dividing it into lesser lengths, &c., which can once more be divided. You will never by division come to "points," _i.e._ mere positions without magnitude of divisibility.