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 1/2B twice the distance G, and in 1/2D it
will move 1/2B 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.