Lecture 5 — Natural motion in natural bodies. Leucippus and Democritus

428 The necessity that each of the simple bodies should have a natural movement may be shown as follows.

429 They manifestly move, and if they have no proper movement they must move by constraint: and the constrained is the same as the unnatural. Now an unnatural movement presupposes a natural movement which it contravenes, and which, however many the unnatural movements, is always one. For naturally a thing moves in one way, while its unnatural movements are manifold.

430 The same may be shown, from the fact of rest. Rest, also, must either be constrained or natural, constrained in a place to which movement was constrained, natural in a place movement to which was natural. Now manifestly there is a body which is at rest at the center. If then this rest is natural to it, clearly motion to this place is natural to it. If, on the other hand, its rest is constrained, what is hindering its motion? Something, which is at rest: but if so, we shall simply repeat the same argument; and either we shall come to an ultimate something to which rest where it is or we shall have an infinite process, which is impossible. The hindrance to its movement, then, we will suppose, is a moving thing—as Empedocles says that it is the vortex which keeps the earth still—: but in that case we ask, where would it have moved to but for the vortex? It could not move infinitely; for to traverse an infinite is impossible, and impossibilities do not happen. So the moving thing must stop somewhere, and there rest not by constraint but naturally. But a natural rest proves a natural movement to the place of rest.

431 Hence Leucippus and Democritus, who say that the primary bodies are in perpetual movement in the void or infinite, may be asked to explain the manner of their motion and the kind of movement which is natural to them.

432 For if the various elements are constrained by one another to move as they do, each must still have a natural movement which the constrained contravenes, and the prime mover must cause motion not by constraint but naturally. If there is no ultimate natural cause of movement and each preceding term in the series is always moved by constraint, we shall have an infinite process.

575. After disproving the position of those who posit that all bodies are generated from surfaces, the Philosopher here begins to inquire whether natural bodies have natural motions. Concerning this he does two things:

first, he shows that natural bodies have natural motions;

secondly, how compulsory motions of bodies take place in ways other than those of natural motions, at but since 'nature' (L. 7; 301b17; [590]).

Regarding the first he does two things:

first he shows that natural bodies have natural motion;

second he shows that they have heaviness and lightness, by which they are inclined to their natural motion, at we go on to show that (301a22).

Regarding the first he does two things:

first he proves that natural bodies have natural motions;

secondly, he refutes the opinions of certain philosophers, in error on this matter, at hence Leucippus (300b8; [578]).

In regard to the first he does two things.

First, he proposes what he intends (300a20), and says that because it was said above that the operations and passions of bodies are their generations and motions, and we have already inquired about the generation of bodies, it remains to discuss their motions. And he says it is manifest from what will be said that there must be some natural motion in all simple bodies, whereas mixed bodies follow the motion of the simple body that is predominant in it. Therefore, some natural motion is in every natural body.

576. Secondly, at they manifestly move (300a21) he proves his proposition with two arguments.

The first of which is based on motion. For we see by the senses that simple bodies are moved; if, therefore, they do not have a proper motion natural to them, then they must be being moved by violence. But to be moved by violence is the same as to be moved beside nature—for what is according to nature is not violent, because the violent is that in which what undergoes the force contributes nothing, as is said in Ethics III. Now from the fact that there are motions beside nature, it follows that there is motion according to nature in respect to which a violent motion is called "beside nature" [praeter naturam]; for sickness would not be a disposition beside nature, unless health were a disposition according to nature, because every privation presupposes something positive. And although from the fact that there is a motion beside nature, it follows that there is a motion according to nature, yet, while there are many motions beside nature, the motion according to nature is one, i.e., for one body, because the nature of one thing is determined to one, from which many deviations are possible, just as health is one but sicknesses many. The reason for this is that each thing is absolutely according to its nature, i.e., in one way, because the nature of one thing is one; but each thing has not only many motions, but also many dispositions, beside nature. But against this seems to be the statement made in the beginning of this book, namely, that to a motion according to nature there is contrary to it one beside nature, and that one thing is contrary to one. To this it can be said that the Philosopher is speaking there of simple motions: for one body cannot be moved beside nature with several simple motions, although it can be moved beside its nature by several composite motions. Or it can be said that even though one thing is contrary to one, yet the contrary which is as privation can occur in many ways, just as health is something absolutely, but sickness can occur in many ways. Similarly, motion according to nature occurs in one way, but beside nature in many ways.

577. The second argument is presented at the same may be shown (300a27), and is based on rest. It presupposes two things.

First of all, that whatever is at rest must be resting either according to violence or according to nature. Secondly, a thing rests through violence in a place whither it is moved through violence; but wherever it is moved according to nature, it rests according to nature.

From these suppositions he argues thus: It is evident to our senses that some body, for example, earth or a stone, is at rest in the middle. Therefore, according to what has been premised, it is resting either through violence or according to nature. If according to nature, then, according to the suppositions, it follows that the motion of such a body to this place is natural. But if it is resting through violence, then something must be present exerting violence upon it and preventing it from being moved. But what is so preventing it from being moved is itself either in motion or at rest. If it is at rest, as a pillar at rest prevents a stone upon it from being moved, the same question returns about this impediment: Is it at rest naturally or violently? If naturally, it will be concluded that it is also moved naturally; but if violently, it will in turn need something preventing it. Consequently, it is necessary either to arrive at some first thing at rest according to nature, which will consequently be moved according to nature, or to go on infinitely in bodies—which is impossible, as was shown in book I. But if it be said that a thing at rest by violence in the middle is prevented from being moved by something in motion (as Empedocles said that the earth is at rest, being prevented by violence from moving by the gyration of the heaven) then, if that obstacle were removed, the consequence would be that the previously impeded body would be carried to some definite place—it being impossible for it to be carried ad infinitum, since it is impossible for the infinite to be traversed, and nothing can be in the process of becoming which cannot come to be. If then it is being moved to some definite place, then, when it shall have arrived there, it will stop or rest there not violently but naturally. And so, according to the premises, if it rests naturally in this place, it follows that it is moved naturally to this place. Consequently, there will exist some natural motion.

578. Then, at hence Leucippus (300b8), he disproves the opinions of certain philosophers on this matter.

First the opinion of Democritus;

secondly, the opinion of Plato, at the same difficulty is involved (L. 6; 300b16).

In regard to the first he does two things.

First, from the premises he concludes to the inadequacy of Democritus' sayings. For he posited that the indivisible bodies which he said to be principles are always being moved in an infinite and empty space. But it has been shown that simple bodies have some natural motion. Therefore, they should decide by what kind of motion such bodies are being moved and what their natural motion is. However, since they have not determined this, their doctrine is incomplete.

579. Secondly, at for if the various elements (300b11) he presents a certain excuse, because they said that one of those indivisible bodies which they posited as elements is moved violently by another. But he rejects this on two grounds.

First, because if violent motion is posited, then natural motion must be posited, beside which the violent motion is, as was said above.

Secondly, because at least the first mover must not cause motion through violence but according to nature. For whatever causes motion through violence has the principle of its motion without, and thus does not move except as moved. If then a first thing causing motion by nature is not posited, but always one that is acting through violence being previously moved by something else, there will be an infinite process in movers—which is impossible, as was proved in Physics VIII. Therefore, they are not excused from the need for positing natural motion.