Aristotle (384-322 BC)

Physics

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

Book VI

Chapter 2

And since every magnitude is divisible into magnitudes—for we have shown that it is impossible for anything continuous to be composed of indivisible parts, and every magnitude is continuous—it necessarily follows that the quicker of two things traverses a greater magnitude in an equal time, an equal magnitude in less time, and a greater magnitude in less time, in conformity with the definition sometimes given of ‘the quicker.’ Suppose that Α is quicker than Β. Now since of two things that which changes sooner is quicker, in the time ΖΗ, in which Α has changed from Γ to Δ, Β will not yet have arrived at Δ but will be short of it: so that in an equal time the quicker will pass over a greater magnitude. More than this, it will pass over a greater magnitude in less time: for in the time in which Α has arrived at Δ, Β being the slower has arrived, let us say, at Ε. Then since Α has occupied the whole time ΖΗ in arriving at Δ, Α will have arrived at Ε in less time than this, say ΖΚ. Now the magnitude ΓΔ that A has passed over is greater than the magnitude ΓΕ, and the time ΖΚ is less than the whole time ΖΗ: so that the quicker will pass over a greater magnitude in less time. And from this it is also clear that the quicker will pass over an equal magnitude in less time than the slower. For since it passes over the greater magnitude in less time than the slower, and (regarded by itself) passes over ΛΜ the greater in more time than ΛΞ the lesser, the time ΠΡ in which it passes over ΛΜ will be more than the time ΠΣ, which it passes over ΛΞ: so that, the time ΠΡ being less than the time ΠΧ in which the slower passes over ΛΞ, the time ΠΣ will also be less than the time ΠΞ: for it is less than the time ΠΡ, and that which is less than something else that is less than a thing is also itself less than that thing. Hence it follows that the quicker will traverse an equal magnitude in less time than the slower. Again, since the motion of anything must always occupy either an equal time or less or more time in comparison with that of another thing, and since, whereas a thing is slower if its motion occupies more time and of equal velocity if its motion occupies an equal time, the quicker is neither of equal velocity nor slower, it follows that the motion of the quicker can occupy neither an equal time nor more time. It can only be, then, that it occupies less time, and thus we get the necessary consequence that the quicker will pass over an equal magnitude (as well as a greater) in less time than the slower.

And since every motion is in time and a motion may occupy any time, and the motion of everything that is in motion may be either quicker or slower, both quicker motion and slower motion may occupy any time: and this being so, it necessarily follows that time also is continuous. By continuous I mean that which is divisible into divisibles that are infinitely divisible: and if we take this as the definition of continuous, it follows necessarily that time is continuous. For since it has been shown that the quicker will pass over an equal magnitude in less time than the slower, suppose that Α is quicker and Β slower, and that the slower has traversed the magnitude ΓΔ in the time ΖΗ. Now it is clear that the quicker will traverse the same magnitude in less time than this: let us say in the time ΖΟ. Again, since the quicker has passed over the whole ΓΔ in the time ΖΟ, the slower will in the same time pass over ΓΚ, say, which is less than ΓΔ. And since Β, the slower, has passed over ΓΚ in the time ΖΟ, the quicker will pass over it in less time: so that the time ΖΟ will again be divided. And if this is divided the magnitude ΓΚ will also be divided just as ΓΔ was: and again, if the magnitude is divided, the time will also be divided. And we can carry on this process for ever, taking the slower after the quicker and the quicker after the slower alternately, and using what has been demonstrated at each stage as a new point of departure: for the quicker will divide the time and the slower will divide the length. If, then, this alternation always holds good, and at every turn involves a division, it is evident that all time must be continuous. And at the same time it is clear that all magnitude is also continuous; for the divisions of which time and magnitude respectively are susceptible are the same and equal.

Moreover, the current popular arguments make it plain that, if time is continuous, magnitude is continuous also, inasmuch as a thing passes over half a given magnitude in half the time taken to cover the whole: in fact without qualification it passes over a less magnitude in less time; for the divisions of time and of magnitude will be the same. And if either is infinite, so is the other, and the one is so in the same way as the other; i.e. if time is infinite in respect of its extremities, length is also infinite in respect of its extremities: if time is infinite in respect of divisibility, length is also infinite in respect of divisibility: and if time is infinite in both respects, magnitude is also infinite in both respects.

Hence Zeno’s argument makes a false assumption in asserting that it is impossible for a thing to pass over or severally to come in contact with infinite things in a finite time. For there are two senses in which length and time and generally anything continuous are called ‘infinite’: they are called so either in respect of divisibility or in respect of their extremities. So while a thing in a finite time cannot come in contact with things quantitatively infinite, it can come in contact with things infinite in respect of divisibility: for in this sense the time itself is also infinite: and so we find that the time occupied by the passage over the infinite is not a finite but an infinite time, and the contact with the infinites is made by means of moments not finite but infinite in number.

The passage over the infinite, then, cannot occupy a finite time, and the passage over the finite cannot occupy an infinite time: if the time is infinite the magnitude must be infinite also, and if the magnitude is infinite, so also is the time. This may be shown as follows. Let ΑΒ be a finite magnitude, and let us suppose that it is traversed in infinite time Γ, and let a finite period ΓΔ of the time be taken. Now in this period the thing in motion will pass over a certain segment of the magnitude: let ΒΕ be the segment that it has thus passed over. (This will be either an exact measure of ΑΒ or less or greater than an exact measure: it makes no difference which it is.) Then, since a magnitude equal to ΒΕ will always be passed over in an equal time, and ΒΕ measures the whole magnitude, the whole time occupied in passing over ΑΒ will be finite: for it will be divisible into periods equal in number to the segments into which the magnitude is divisible. Moreover, if it is the case that infinite time is not occupied in passing over every magnitude, but it is possible to pass over some magnitude, say ΒΕ, in a finite time, and if this ΒΕ measures the whole of which it is a part, and if an equal magnitude is passed over in an equal time, then it follows that the time like the magnitude is finite. That infinite time will not be occupied in passing over ΒΕ is evident if the time be taken as limited in one direction: for as the part will be passed over in less time than the whole, the time occupied in traversing this part must be finite, the limit in one direction being given. The same reasoning will also show the falsity of the assumption that infinite length can be traversed in a finite time. It is evident, then, from what has been said that neither a line nor a surface nor in fact anything continuous can be indivisible.

This conclusion follows not only from the present argument but from the consideration that the opposite assumption implies the divisibility of the indivisible. For since the distinction of quicker and slower may apply to motions occupying any period of time and in an equal time the quicker passes over a greater length, it may happen that it will pass over a length twice, or one and a half times, as great as that passed over by the slower: for their respective velocities may stand to one another in this proportion. Suppose, then, that the quicker has in the same time been carried over a length one and a half times as great as that traversed by the slower, and that the respective magnitudes are divided, that of the quicker, the magnitude ΑΒΓΔ, into three indivisibles, and that of the slower into the two indivisibles ΕΖ, ΖΗ. Then the time may also be divided into three indivisibles, for an equal magnitude will be passed over in an equal time. Suppose then that it is thus divided into ΚΛ, ΛΜ, ΜΝ. Again, since in the same time the slower has been carried over ΕΖ, ΖΗ, the time may also be similarly divided into two. Thus the indivisible will be divisible, and that which has no parts will be passed over not in an indivisible but in a greater time. It is evident, therefore, that nothing continuous is without parts.

Book VI Chapter 1

Book VI Chapter 3