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Metaphysics   


regards quantity one kind is 'complete' and the other 'incomplete',
and as regards spatial movement one is 'upwards' and the other
'downwards', or one thing is 'light' and another 'heavy'); so that
there are as many kinds of movement and change as of being. There
being a distinction in each class of things between the potential
and the completely real, I call the actuality of the potential as
such, movement. That what we say is true, is plain from the
following facts. When the 'buildable', in so far as it is what we mean
by 'buildable', exists actually, it is being built, and this is the
process of building. Similarly with learning, healing, walking,
leaping, ageing, ripening. Movement takes when the complete reality
itself exists, and neither earlier nor later. The complete 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

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