51. Celestial Substances and Terrestrial Bodies. -- A grandiose and imposing spectacle confronts us in the regular revolutions of the heavens and the seeming immutability of the stars. Aristotle held the substance of the stars to be of a nobler order than that of the earth, -- influenced, no doubt, by the popular superstition which regarded them as gods. And this distinction accounts for the various sections of his special physics: celestial substances, sublunary bodies, and the action of the former on the latter.

(1) The Celestial Substance. -- Its perfection is evidenced by its local motion and by its inner constitution.

The motion of the heavenly bodies is circular motion. This is the most perfect of all motions, for it has neither beginning, middle, nor end; hence it is the only motion that is eternal. Circular rotation is uniform, and hence invariable, like the action of the Prime Mover on whom it depends. And since all substantial change supposes a certain opposition between starting-point and term, it follows that the heavenly bodies cannot pass through contrary states: they are above and beyond all change, immutable, unproducible, incorruptible. The peculiar element of which they are composed he calls ether, a substance purely topical in its nature, hulên . . . monon kata topon kinêtên (Metaph., viii., 4, 1044 b), and possessing nothing in common with the matter of the terrestrial elements.

The stars are fixed upon one single sphere, and their daily motion round the earth is performed in the same time as that of the sphere to which they belong. On the other hand, in order to explain the intricate motion of the planets it was found necessary to consider them as attached to different spheres.^{1} Comets were regarded as aerial will-o'-the-wisps. The internal motive-power of each sphere is an eternal intelligence, a principle of the mental order, ever tending towards the Prime Mover. The most perfect sphere is the highest, remotest of the whole firmament (prôtos ouranos), or the sphere of the fixed stars, because it is nearest the Prime Mover and farthest from the earth. The relations of these motor-intelligences with the world-soul and with the Prime Mover are not clearly defined.

(2) The Sublunary or Terrestrial Body. -- The four terrestrial elements are: earth in the centre -- absolutely solid and heavy -- water which surrounds the earth, air which surrounds the water, and fire -- absolutely light -- in the higher altitudes. Each of these elements has a natural rectilinear motion (upwards and downwards respectively) and tends towards a natural position (locus naturalis), which is at once its form and its end; to become fire and to move upwards, to become earth and to move downwards, are for Aristotle one and the same thing.^{2} Owing to their mutual opposition as regards motion and as regards their sensible qualities (the active couple, hot and cold, and the passive couple, dry and wet, may be united disjunctively, and so give rise to a fourfold combination), the four elements can explain, by their changes, combinations, and mixtures, the formation of sublunary bodies.

(3) The Action of the Heavens on Terrestrial Bodies. -- The sky, being the source of the earth's motion, is also the source of all sublunary generation or change. The immediate cause of the latter is the heat produced by the friction of the astral spheres with the atmosphere or the upper part of the terrestrial world. This friction changes the air into fire. The ecliptic declination of the sun explains, by its periodical approach to, and withdrawal from, certain parts of the earth, the rhythmical alternation of generation and dissolution in existing things. From the absence of a vacuum, from the oneness of the primary circular movement, and from the tendency which draws all the portions of the same elements towards the same place, Aristotle infers the unity of the world. From the accumulation and the sinking of the various parts of the earth, he deduces the geocentric theory of the universe. From the perfectly spherical system in which everything is contained he concludes that the world is finite. And we know already that in his view the world is eternal like motion, Nature, and the Prime Mover.

Among sublunary bodies, living organisms, and man above all, take a special place: this brings us to Aristotle's psychology.

{1} Aristotle held the opinions of Eudoxus and Callippus in astronomy.

{2} PIAT, op. cit., p. 108.