=The Mechanical Series,――Aristotle’s Theory of Physics.= The general astronomical assumptions of the time determined Aristotle’s theory of the physical world. He adopted the old Pythagorean conception of the limited world-all: a hollow sphere made up of concentric crystalline spheres. In opposition to the Pythagoreans, he conceived the earth at the centre. It is spherical and stationary. Around it the crystalline spheres revolve, in which the moon, sun, five planets, and fixed stars are placed. The fixed stars are in the rim of the great sphere, are outside all, and are nearest therefore to God, who animates all. God as it were holds the world-all in the hollow of his hand. He moves the whole, which in turn moves the fifty-five concentric crystal spheres within. The principle of the movement of fixed stars is that of the Deity, while the principle of the other spheres is that of the spirits which reside in them. The movement of the planets have an influence upon terrestrial life. Aristotle made the usual Pythagorean division between the celestial and the terrestrial parts of the world-all, which has had so much influence upon theology. The motion of the world-all is most perfect, being a circle; its form is most perfect, being a sphere. The celestial part of this world-all, which is the region lying near the periphery, is most like God. The motion of this heaven is circular, and it is the place of uniformity, perfectness, and changeableness. The stars do not change nor pass away. They are superhuman beings, who in their regularity are like the blessed gods. The terrestrial part of the world-all below the moon has motions in straight lines. This is the theatre of imperfection and irregularity, of increase and diminution. There are many interesting discussions by Aristotle upon particular physical matters, such as space, time, the elements. His conception of motion shows how the series of uniform nature-motions lead up to the second series of teleological values. In nature there are three kinds of motion: change of place (mechanical); change in quality (chemical); change in substance (organic). While change of place is the lowest kind of motion, it is necessary to chemical and organic changes. Yet Aristotle refuses to allow that qualitative changes can be reduced to quantitative changes, but maintains that quality is self-subsistent. Organic change, or change in substance, on the contrary, has a higher Form of reality than the lower changes. This stand taken by Aristotle, in refusing to reduce qualitative to quantitative determinations, shows how comprehensive and sane a scientist he was. It introduces us to a psychology and an ethics that are intimately linked to physics, and at the same time have realms of their own. Let us now turn to the series of qualitative nature changes, or to psychology, ethics, and politics. =The Teleological Series: The Qualitative Changes of Phenomena.=