Aristotle's study of mobile being is not all in the Physics, which is only a general treatise on motion and its principles. The particular kinds of motion and the special problems relating to them are the burden of separate works following the Physics, notably of De Caelo, De Generatione et Corruptione, and the Meteorologica. These works are of interest mainly to the specialist or, perhaps, the antiquarian. There is in them constant reference to scientific conceptions current in Aristotle's time but long since obsolete. This not only adds to the reader's task but also limits the advantage to be gained from them. Nevertheless, the dedicated scholar will study them.
Yet even the general student should have some acquaintance, if not with all the topics of these special treatises, at least with one - I mean Aristotle's astronomy. The influence of this astronomy, while perhaps not so complete as sometimes thought, was still very great, abiding moreover close on to two thousand years. In this last chapter before our concluding appraisal we shall, accordingly, trace the basic design of Aristotle's astronomical universe and then indicate the modern innovations that were to render it a thing of the past.1
1. Aristotle's Astronomical System
a) Basic postulates. - All the astronomers of antiquity agreed that their hypotheses had not only to be scientifically (or mathematically) correct and as simple as possible, but also capable of explaining the "look" of celestial motions. In the expression which Simplicius attributes to Plato, any acceptable astronomical theory had "to save the appearances" - CTQEGV Ta ckatvOlieva. This, to the astronomers, was the basic test, and all their efforts were aimed at meeting it by resolving the apparent travelings of the celestial bodies into the simplest motions possible. But if all of them pursued this aim, not all claimed the same degree of objective reality for their theories. On this basis they classify into two groups. One group, the mathematicians, cared little whether their theories of celestial mechanics were actually embodied in nature. The other, the physicists, were convinced that the theories devised by them were factual arrangements, and not just mathematical inventions. Believing that their system or systems copied reality,they had therefore the greater burden; unlike themathematicians, they had also to show how nature operates the system, how one part of the heavens moves another, and the whole moves together. Aristotle, a physicist, certainly numbers in the second group, those who believed that their theories were actual fact and not just mathematical, though serviceable, figment.
Pursuing this goal of tallying theory with reality, the physicists set to work. Underlying their efforts were two very important assumptions, briefly stated as follows:
- the heavenly bodies, being perfect, are moved with perfect motion, which, to recall, is circular and uniform;
- the earth, in the shape of a sphere, is at rest at the center of the world or universe, the latter being visualized as an immense shell of finite dimensions.
On the foundation of these two premises, and with considerable borrowing from the Platonists Eudoxus and Callippus, Aristotle built his astronomical system, a system of concentric spheres.
b) The astronomy of concentric spheres. - According to Aristotle, the universe is to be conceived as an interlocking system of concentric spheres, with the earth as their common center. Their radius, however, varies, increasing with each successive sphere. The stars, having no motion themselves, are carried by the spheres, and are moved with their motion. The outermost sphere makes a uniform rotation on its axis once in twenty-four hours, and attached to it are the so-called fixed stars.
The seven planets then known - Saturn, Jupiter, Mars, Venus, Mercury, the sun, and moon - were carried by intermediate spheres. But since the movement of one sphere could not describe the irregularities observable in the orbits of the planets, each planetary trajectory was accounted for by a combination of several circular motions. Thus, each planet had its individual system of spheres - five, says Aristotle - so arranged that the poles of one were joined at the right places to the next immediate sphere. Since, moreover, Aristotle meant his system to be successful in fact as well as in theory, he was faced with further complicating his mechanics of the heavenly bodies. Not to go into detail, suffice it to say that he had to introduce compensating spheres which canceled out certain movements within each planetary subsystem.2
All told, then, he gets fifty-five spheres, though at times he places the number at forty-nine. For Aristotle, to repeat, the spheres were real; they existed in the heavens, or rather they were the heavens, and consisted of an incorruptible transparent element, ether, the so-called fifth element altogether different from the four terrestrial elements. The first heaven, that is to say the outermost sphere of the universe, was moved by the prime mover, the other spheres by movers distinct from the prime mover. But the relation of these second movers to one another and to the prime mover is not made very clear. Apparently, the movers of the lower spheres were to be understood as souls that desired and, by desiring, imitated as far as possible the eternal life of the prime mover. This much, then, for Aristotle's celestial mechanics.
c) Composition and movements of the sublunary world. - In Aristotle's view, as we have learned, the world we inhabit is made of four elementary bodies: water, air, earth, and fire, a classification that was accepted for centuries after him. These elements have a tendency to move up or down, following the attraction of their natural place. The motion of the elements puts them in contact with one another, thus making possible the alterations corresponding to their basic contrarieties. Subsequent to alterations, assuming the proper moment, are substantial generations and corruptions. But commanding the alternating rhythm of all these transformations is the movement of the sun, which, following the ecliptic, is successively nearer and farther away and thereby exercises a varying influence on the earth and all that is in it. So it is that in Aristotle's account the whole sublunary world, the life and activity of each of the beings that compose it, appear in the final reckoning to be regulated by the movement of the heavenly bodies, of the sun in particular.
Such is Aristotle's conception of the world and all that moves in it, a system relatively simple and remarkably coherent. Yet in this system are combined an astronomy, a physics, and even, to use the modern word, a chemistry, the chemistry of the elementary bodies and also of their transformations. But its most striking feature is the contrast between the constancy of the heavens and the instability of sublunar bodies, between the basic immutability of the one and the essential perishableness of the other. Celestial bodies, incorruptible and ingenerable, receive only uniform, circular motion, whereas terrestrial bodies are open to every manner of change and decay. Modern scientists were to find it otherwise. Theirs, it was soon apparent, would be a universe without this radical difference between the globe of man and the expanse out yonder. The same system of motion would govern the whole, and the same general kind of properties, activities, and defectibilities inhabit the whole.