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On this principle of principles depend the heavens and nature. 1


According to Aristotle, the sky is the perfect sphere of which the earth is supposed to be the centre ; nature is everything within this sphere that is subject to motion or to rest; or, more abstractly, it is motion itself, in so far as the latter emanates from the first mover and is continued by the secondary causes. Physics is a theory of motion.2 It inquires into the immovable principle (the divine), the imperishable moving power (the heaven), and the perishable world or sublunary nature. There are as many kinds of movement as there are categories of being.4 The principal ones are: (1) movement that affects the substance, or origin and decay (yéveois kai popa); (2) movement that affects the quality, or change of quality, alteration (kívnois καταλλοίωσιν, μεταβολή); (3) movement that affects the quantity, or addition and subtraction (Kívnous kat av&nouv kai po loiv); (4) local movement, or change of place (dopá, kívnous catà TÒV Tómová). The first (origin and decay), however, is not, strictly speaking, a movement, while, of the other three, change of place is regarded by all the physicists, and especially by Anaxagoras, as the most important, the most universal, and the most original form of motion. Motion, change, energy, or entelechy, is the realization of the potential as such.7 But it is not a substance (ovo ía), and does not exist apart from the things which it affects (παρά τα πράγματα).

1 Μet., ΧΙΙ., 7, 11: 'Εκ τοιαύτης άρα αρχής ήρτηται ο ουρανός και η φύσις. 2 Phys., III., 1, 1.

s ld., II., 7.

Id., III., 1, 2. 6 Id., III., 1, 7.

6 ld., VIII., 10. * Id., IΙI., 1, 7: “Η του δυνάμει όντος εντελέχεια.



Space (xópa, TÓTOS) is more like a substance. It is, however, neither the material of which bodies are made, as Plato erroneously supposes in the Timæus,' nor their form, nor the interval which separates them (Sidornua), but the limit between the surrounding and the surrounded body,2 between the contents and the container. This singular definition is intended by Aristotle as a disavowal of the conception that there is such a thing as empty space separating bodies from each other (the kevóv of Democritus), a view which he regarded as erroneous. Movement, according to him, does not imply the existence of the void ; it is invariably a change of place of different bodies. The condensation of a body presupposes the rarefaction of the surrounding body, and vice versa. Consequently, there is no void either in the bodies or outside of them. Since space cannot be conceived without movement, the immovable (the divine) is not in space. Moreover, inasmuch as space is the boundary between the container and the contained, and since the universal is not contained in anything, but contains everything, the universe or the All cannot occupy a particular place. Hence the universe, or the whole of things, does not, strictly speaking, move. Its parts alone suffer a change of place. Taken as a whole, however, it can only revolve upon itself. Indeed, certain portions of the heavens move, not upwards and downwards, but in a circle, and only the denser or lighter substances are carried downwards and upwards.4

Like space, time exists only as the condition of motion ; it is the measure or number of motion. It is potentially infinite like motion (whatever Plato may say of it), and this distinguishes it from space which is limited. It is nonsense to speak of an actually infinite space. Infinity is merely potential and never actual ; for the actual has

· Phys., IV., 1. 2 Id., IV., 6: Το πέρας του περιέχοντος σώματος Id., IV., 8.

* ld., IV., 7, 5.


; form ; it is determined or finite; the potential is not finite, but infinite. Conversely, infinity has potential existence only in the infinite multiplication of numbers and the infinite divisibility of magnitudes. Now, time is the measure of motion and consequently a number, and number presupposes a person who can count. Hence it follows that time presupposes a soul and cannot exist except for a numbering soul.1

We distinguished between several kinds of movement, the most important of which is called change of place. The latter, again, has different forms. The first and the most perfect of these is movement in a circle, which is the only motion that can be endless, simple, and uniform. Rectilinear motion cannot be constant, and is therefore less perfect than the other. It cannot be continued ad infinitum, because Aristotle's universe is limited; hence, in order to continue, it must return upon itself or become oscillatory; and there is bound to be a stop, however minimal it may be, at the point where the movement begins again to go in the opposite direction.

Circular movement and rectilinear movement upward and downward are the two great forms of kívnous in the physical world. The former, which is the most perfect, because it is simple and continuous, belongs to the highest heavens (TTPÔTOS oúpavos), the solid vault which supports the fixed stars ; 2 the latter, which is less perfect because it is not absolutely continuous, moves the lower or central parts of the universe. The eternal revolution of the outermost heavens around the axis of the world is immediately caused by the immovable first mover, who moves the other parts of the world only indirectly and by means of the πρώτος ουρανός. TT PÛtos oúpavós. Hence, the sphere of the fixed stars is in the πρώτον κινούν κινούμενον, the first moved mover, and communicaces its motion to the lower or planetary spheres (dettepos oúpavos). These solid but transparent spheres, of which there are about fifty, revolve around a common centre, the centre of the earth, which is also the centre of the world. But their movement is no longer a simple movement; they rotate from left to right, like the outermost heaven, but they also move from right to left. This com. plicated movement can only be explained on the assumption that each sphere has, in addition to the first moved mover, a particular, relatively-independent mover. Finally, the central sphere, that is, the earth and its inhabitants, its ocean, and its two atmospheres, is placed under the direct guidance of the planets and under the indirect influence of the fixed stars. It does not revolve around its own axis, but executes complex movements, the fundamental form of which is upward and downward movement.

1 Phys., IV., 20, 4.

? The modern theory of heavenly bodies moving in space, a view which prevailed among the Ionians and the Pythagoreans, seems to be wholly foreign to Aristotle. When he speaks of the heaven and its motion, he does not mean, by metonymy, the motion of the stars enclosed in this space; his idea is that the heaven itself, that is, the entire series of concentric spheres, which consist of the same substance as their stars, moves. He also likens the motion of the stars to the movement of a person seated in a chariot; the person is immov. able and yet advances as the chariot advances.

Things that move downwards from the universal circumference to the universal centre are called heavy; things that move upwards from the earth towards the sky are called light. The opposition between heavy and light is the same as that between cold and warm ; for experience shows that cold air falls and warm air rises, On this double opposition depends the differentiation of elements. Heavy and cold matter forms the earthy or solid element; light and warm matter produces fire. Water and air, that is, moisture and dryness, form two intermediate elements, whose purpose is to reconcile the contrary extremes. Although Aristotle thus assumes the fur otoixeia of

Empedocles, he maintains with Heraclitus and Democritus that these elements are homogeneous, and that they represent successive transformations of one and the same matter. In fact, experience shows him that solids pass into liquids, liquids into gases, gases into fire, and vice versa, that fire and gases are liquefied, and liquids solidified. Hence, he identifies the chemical notion of element with the physical notion of state.

The difference existing between the elements of sublunary matter depends essentially on the nature of the movement peculiar to the earth, and does not extend beyond our world. It is not found in the celestial spheres, which consist of pure ether. This ether is not a fifth element (TTÉUTITOV Otoixelov), as has been erroneously believed, but the original and neutral substance which Anaximander called the atteipov, and whicli is the substratum common to the four elements of the terrestrial sphere. There can be no dense liquid, gaseous, or fiery elements in the heavens, because there is no contrast between heavy and light, cold and warm, in that region; and this contrast does not exist in the heavenly spheres, because rectilinear and vertical motion is unknown there.

Removed as they are from the contrasts of our perishable world, and coming into direct communion with the first mover, who dwells in the outermost heaven,' the bright inhabitants of the skies enjoy happiness unalloyed, and are endowed with immortality. They of all beings most resemble the unmoved first mover. Their movements are not arbitrary; what seems to be an imperfection is in reality a divine prerogative. Even the free man is much more determined in his actions than the slave and the animal; for he obeys the established laws of the State, while they contribute but little to public affairs, and habitually act by chance. The more reason a being possesses, the more reg 1 Phys., VIII., 14, 24.

2 Met., XII., 10, 4.

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