The Complete Aristotle (eng.)

reality, then, of that which exists
potentially, when it is completely real and actual, not qua itself,
but qua movable, is movement. By qua I mean this: bronze is
potentially a statue; but yet it is not the complete reality of
bronze qua bronze that is movement. For it is not the same thing to
be bronze and to be a certain potency. If it were absolutely the
same in its definition, the complete reality of bronze would have
been a movement. But it is not the same. (This is evident in the
case of contraries; for to be capable of being well and to be
capable of being ill are not the same-for if they were, being well
and being ill would have been the same-it is that which underlies
and is healthy or diseased, whether it is moisture or blood, that
is one and the same.) And since it is not. the same, as colour and
the visible are not the same, it is the complete reality of the
potential, and as potential, that is movement. That it is this, and
that movement takes place when the complete reality itself exists,
and neither earlier nor later, is evident. For each thing is
capable of being sometimes actual, sometimes not, e.g. the
buildable qua buildable; and the actuality of the buildable qua
buildable is building. For the actuality is either this-the act of
building-or the house. But when the house exists, it is no longer
buildable; the buildable is what is being built. The actuality,
then, must be the act of building, and this is a movement. And the
same account applies to all other movements.
    That what we have said is right is evident from what all others
say about movement, and from the fact that it is not easy to define
it otherwise. For firstly one cannot put it in any class. This is
evident from what people say. Some call it otherness and inequality
and the unreal; none of these, however, is necessarily moved, and
further, change is not either to these or from these any more than
from their opposites. The reason why people put movement in these
classes is that it is thought to be something indefinite, and the
principles in one of the two ‘columns of contraries’ are indefinite
because they are privative, for none of them is either a ‘this’ or
a ‘such’ or in any of the other categories. And the reason why
movement is thought to be indefinite is that it cannot be classed
either with the potency of things or with their actuality; for
neither that which is capable of being of a certain quantity, nor
that which is actually of a certain quantity, is of necessity
moved, and movement is thought to be an actuality, but incomplete;
the reason is that the potential, whose actuality it is, is
incomplete. And therefore it is hard to grasp what movement is; for
it must be classed either under privation or under potency or under
absolute actuality, but evidently none of these is possible.
Therefore what remains is that it must be what we said-both
actuality and the actuality we have described-which is hard to
detect but capable of existing.
    And evidently movement is in the movable; for it is the complete
realization of this by that which is capable of causing movement.
And the actuality of that which is capable of causing movement is
no other than that of the movable. For it must be the complete
reality of both. For while a thing is capable of causing movement
because it can do this, it is a mover because it is active; but it
is on the movable that it is capable of acting, so that the
actuality of both is one, just as there is the same interval from
one to two as from two to one, and as the steep ascent and the
steep descent are one, but the being of them is not one; the case
of the mover and the moved is similar.
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10
    The infinite is either that which is incapable of being
traversed because it is not its nature to be traversed (this
corresponds to the sense in which the voice is ‘invisible’), or
that which admits only of incomplete traverse or scarcely admits of
traverse, or that which, though it naturally admits of traverse, is
not traversed or limited; further, a thing may be infinite in
respect of addition or of subtraction, or both. The infinite cannot
be a separate, independent thing. For if it is neither a spatial
magnitude nor a plurality, but infinity itself is its substance and
not an accident of it, it will be indivisible; for the divisible is
either magnitude or plurality. But if indivisible, it is not
infinite, except as the voice is invisible; but people