**Aristotle, Physics VII**

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**Part 5**

Now since wherever there is a movent, its motion always acts upon something, is
always in something, and always extends to something (by 'is always in
something' I mean that it occupies a time: and by 'extends to something' I mean
that it involves the traversing of a certain amount of distance: for at any
moment when a thing is causing motion, it also has caused motion, so that there
must always be a certain amount of distance that has been traversed and a
certain amount of time that has been occupied). then, A the movement have moved
B a distance G in a time D, then in the same time the same force A will move
1/2 B twice the distance G, and in 1/2 D it will move 1/2 B the whole distance
for G: thus the rules of proportion will be observed. Again if a given force
move a given weight a certain distance in a certain time and half the distance
in half the time, half the motive power will move half the weight the same
distance in the same time. Let E represent half the motive power A and Z half
the weight B: then the ratio between the motive power and the weight in the one
case is similar and proportionate to the ratio in the other, so that each force
will cause the same distance to be traversed in the same time.

But if E move Z a distance G in a time D, it does not
necessarily follow that E can move twice Z half the distance G in the same
time. If, then, A move B a distance G in a time D, it does not follow that E,
being half of A, will in the time D or in any fraction of it cause B to
traverse a part of G the ratio between which and the whole of G is
proportionate to that between A and E (whatever fraction of AE may be): in fact
it might well be that it will cause no motion at all; for it does not follow
that, if a given motive power causes a certain amount of motion, half that
power will cause motion either of any particular amount or in any length of
time: otherwise one man might move a ship, since both the motive power of the ship-haulers
and the distance that they all cause the ship to traverse are divisible into as
many parts as there are men. Hence Zeno's reasoning is false when he argues
that there is no part of the millet that does not make a sound: for there is no
reason why any such part should not in any length of time fail to move the air
that the whole bushel moves in falling. In fact it does not of itself move even
such a quantity of the air as it would move if this part were by itself: for no
part even exists otherwise than potentially.

If on the other hand we have two forces each of which separately moves one of
two weights a given distance in a given time, then the forces in combination
will move the combined weights an equal distance in an equal time: for in this
case the rules of proportion apply.

Then does this hold good of alteration and of increase also? Surely it does,
for in any given case we have a definite thing that cause increase and a definite
thing that suffers increase, and the one causes and the other suffers a certain
amount of increase in a certain amount of time. Similarly we have a definite
thing that causes alteration and a definite thing that undergoes alteration,
and a certain amount, or rather degree, of alteration is completed in a certain
amount of time: thus in twice as much time twice as much alteration will be
completed and conversely twice as much alteration will occupy twice as much
time: and the alteration of half of its object will occupy half as much time
and in half as much time half of the object will be altered: or again, in the
same amount of time it will be altered twice as much.

On the other hand if that which causes alteration or increase causes a certain
amount of increase or alteration respectively in a certain amount of time, it
does not necessarily follow that half the force will occupy twice the time in altering
or increasing the object, or that in twice the time the alteration or increase
will be completed by it: it may happen that there will be no alteration or
increase at all, the case being the same as with the weight.