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Metaphysics   


cases if things which naturally have a quality lose it by violence, we
say they have suffered privation.
2

Since some such originative sources are present in soulless
things, and others in things possessed of soul, and in soul, and in
the rational part of the soul, clearly some potencies will, be
non-rational and some will be non-rational and some will be
accompanied by a rational formula. This is why all arts, i.e. all
productive forms of knowledge, are potencies; they are originative
sources of change in another thing or in the artist himself considered
as other.
And each of those which are accompanied by a rational formula is
alike capable of contrary effects, but one non-rational power produces
one effect; e.g. the hot is capable only of heating, but the medical
art can produce both disease and health. The reason is that science is
a rational formula, and the same rational formula explains a thing and
its privation, only not in the same way; and in a sense it applies
to both, but in a sense it applies rather to the positive fact.
Therefore such sciences must deal with contraries, but with one in
virtue of their own nature and with the other not in virtue of their
nature; for the rational formula applies to one object in virtue of
that object's nature, and to the other, in a sense, accidentally.
For it is by denial and removal that it exhibits the contrary; for the
contrary is the primary privation, and this is the removal of the
positive term. Now since contraries do not occur in the same thing,
but science is a potency which depends on the possession of a rational
formula, and the soul possesses an originative source of movement;
therefore, while the wholesome produces only health and the
calorific only heat and the frigorific only cold, the scientific man
produces both the contrary effects. For the rational formula is one
which applies to both, though not in the same way, and it is in a soul
which possesses an originative source of movement; so that the soul
will start both processes from the same originative source, having
linked them up with the same thing. And so the things whose potency is
according to a rational formula act contrariwise to the things whose
potency is non-rational; for the products of the former are included
under one originative source, the rational formula.
It is obvious also that the potency of merely doing a thing or
having it done to one is implied in that of doing it or having it done
well, but the latter is not always implied in the former: for he who
does a thing well must also do it, but he who does it merely need
not also do it well.
3

There are some who say, as the Megaric school does, that a thing
'can' act only when it is acting, and when it is not acting it
'cannot' act, e.g. that he who is not building cannot build, but
only he who is building, when he is building; and so in all other
cases. It is not hard to see the absurdities that attend this view.
For it is clear that on this view a man will not be a builder
unless he is building (for to be a builder is to be able to build),
and so with the other arts. If, then, it is impossible to have such
arts if one has not at some time learnt and acquired them, and it is
then impossible not to have them if one has not sometime lost them
(either by forgetfulness or by some accident or by time; for it cannot
be by the destruction of the object, for that lasts for ever), a man
will not have the art when he has ceased to use it, and yet he may
immediately build again; how then will he have got the art? And
similarly with regard to lifeless things; nothing will be either
cold or hot or sweet or perceptible at all if people are not
perceiving it; so that the upholders of this view will have to
maintain the doctrine of Protagoras. But, indeed, nothing will even
have perception if it is not perceiving, i.e. exercising its

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