Aristotle (384-322 BC)

Physics

Translated by R. P. Hardie and R. K. Gaye

Book VII

Chapter 1

Everything that is in motion must be moved by something. For if it has not the source of its motion in itself it is evident that it is moved by something other than itself, for there must be something else that moves it. If on the other hand it has the source of its motion in itself, let ΑΒ be taken to represent that which is in motion essentially of itself and not in virtue of the fact that something belonging to it is in motion. Now in the first place to assume that ΑΒ, because it is in motion as a whole and is not moved by anything external to itself, is therefore moved by itself—this is just as if, supposing that ΚΛ is moving ΛΜ and is also itself in motion, we were to deny that ΚΜ is moved by anything on the ground that it is not evident which is the part that is moving it and which the part that is moved. In the second place that which is in motion without being moved by anything does not necessarily cease from its motion because something else is at rest, but a thing must be moved by something if the fact of something else having ceased from its motion causes it to be at rest. Thus, if this is accepted, everything that is in motion must be moved by something. For ΑΒ, which has been taken to represent that which is in motion, must be divisible since everything that is in motion is divisible. Let it be divided, then, at Γ. Now if ΒΓ is not in motion, then ΑΒ will not be in motion: for if it is, it is clear that ΑΓ would be in motion while ΒΓ is at rest, and thus ΑΒ cannot be in motion essentially and primarily. But ex hypothesi ΑΒ is in motion essentially and primarily. Therefore if ΓΒ is not in motion ΑΒ will be at rest. But we have agreed that that which is at rest if something else is not in motion must be moved by something. Consequently, everything that is in motion must be moved by something: for that which is in motion will always be divisible, and if a part of it is not in motion the whole must be at rest.

Since everything that is in motion must be moved by something, let us take the case in which a thing is in locomotion and is moved by something that is itself in motion, and that again is moved by something else that is in motion, and that by something else, and so on continually: then the series cannot go on to infinity, but there must be some first movent. For let us suppose that this is not so and take the series to be infinite. Let Α then be moved by Β, Β by Γ, Γ by Δ, and so on, each member of the series being moved by that which comes next to it. Then since ex hypothesi the movent while causing motion is also itself in motion, and the motion of the moved and the motion of the movent must proceed simultaneously (for the movent is causing motion and the moved is being moved simultaneously) it is evident that the respective motions of Α, Β, Γ, and each of the other moved movents are simultaneous. Let us take the motion of each separately and let Ε be the motion of Α, Ζ of Β, and Η and Θ respectively the motions of Γ and Δ: for though they are all moved severally one by another, yet we may still take the motion of each as numerically one, since every motion is from something to something and is not infinite in respect of its extreme points. By a motion that is numerically one I mean a motion that proceeds from something numerically one and the same to something numerically one and the same in a period of time numerically one and the same: for a motion may be the same generically, specifically, or numerically: it is generically the same if it belongs to the same category, e.g. substance or quality: it is specifically the same if it proceeds from something specifically the same to something specifically the same, e.g. from white to black or from good to bad, which is not of a kind specifically distinct: it is numerically the same if it proceeds from something numerically one to something numerically one in the same period of time, e.g. from a particular white to a particular black, or from a particular place to a particular place, in a particular period of time: for if the period of time were not one and the same, the motion would no longer be numerically one though it would still be specifically one.

We have dealt with this question above. Now let us further take the time in which Α has completed its motion, and let it be represented by Κ. Then since the motion of Α is finite the time will also be finite. But since the movents and the things moved are infinite, the motion ΕΖΗΘ, i.e. the motion that is composed of all the individual motions, must be infinite. For the motions of Α, Β, and the others may be equal, or the motions of the others may be greater: but assuming what is conceivable, we find that whether they are equal or some are greater, in both cases the whole motion is infinite. And since the motion of Α and that of each of the others are simultaneous, the whole motion must occupy the same time as the motion of Α: but the time occupied by the motion of Α is finite: consequently the motion will be infinite in a finite time, which is impossible.

It might be thought that what we set out to prove has thus been shown, but our argument so far does not prove it, because it does not yet prove that anything impossible results from the contrary supposition: for in a finite time there may be an infinite motion, though not of one thing, but of many: and in the case that we are considering this is so: for each thing accomplishes its own motion, and there is no impossibility in many things being in motion simultaneously. But if (as we see to be universally the case) that which primarily is moved locally and corporeally must be either in contact with or continuous with that which moves it, the things moved and the movents must be continuous or in contact with one another, so that together they all form a single unity: whether this unity is finite or infinite makes no difference to our present argument; for in any case since the things in motion are infinite in number the whole motion will be infinite, if, as is theoretically possible, each motion is either equal to or greater than that which follows it in the series: for we shall take as actual that which is theoretically possible. If, then, Α, Β, Γ, Δ form an infinite magnitude that passes through the motion ΕΖΗΘ in the finite time Κ, this involves the conclusion that an infinite motion is passed through in a finite time: and whether the magnitude in question is finite or infinite this is in either case impossible. Therefore the series must come to an end, and there must be a first movent and a first moved: for the fact that this impossibility results only from the assumption of a particular case is immaterial, since the case assumed is theoretically possible, and the assumption of a theoretically possible case ought not to give rise to any impossible result.

Book VI Chapter 10

Book VII Chapter 2