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Metaphysics   


things are prior in substance to perishable things, and no eternal
thing exists potentially. The reason is this. Every potency is at
one and the same time a potency of the opposite; for, while that which
is not capable of being present in a subject cannot be present,
everything that is capable of being may possibly not be actual.
That, then, which is capable of being may either be or not be; the
same thing, then, is capable both of being and of not being. And
that which is capable of not being may possibly not be; and that which
may possibly not be is perishable, either in the full sense, or in the
precise sense in which it is said that it possibly may not be, i.e. in
respect either of place or of quantity or quality; 'in the full sense'
means 'in respect of substance'. Nothing, then, which is in the full
sense imperishable is in the full sense potentially existent (though
there is nothing to prevent its being so in some respect, e.g.
potentially of a certain quality or in a certain place); all
imperishable things, then, exist actually. Nor can anything which is
of necessity exist potentially; yet these things are primary; for if
these did not exist, nothing would exist. Nor does eternal movement,
if there be such, exist potentially; and, if there is an eternal
mobile, it is not in motion in virtue of a potentiality, except in
respect of 'whence' and 'whither' (there is nothing to prevent its
having matter which makes it capable of movement in various
directions). And so the sun and the stars and the whole heaven are
ever active, and there is no fear that they may sometime stand
still, as the natural philosophers fear they may. Nor do they tire
in this activity; for movement is not for them, as it is for
perishable things, connected with the potentiality for opposites, so
that the continuity of the movement should be laborious; for it is
that kind of substance which is matter and potency, not actuality,
that causes this.
Imperishable things are imitated by those that are involved in
change, e.g. earth and fire. For these also are ever active; for
they have their movement of themselves and in themselves. But the
other potencies, according to our previous discussion, are all
potencies for opposites; for that which can move another in this way
can also move it not in this way, i.e. if it acts according to a
rational formula; and the same non-rational potencies will produce
opposite results by their presence or absence.
If, then, there are any entities or substances such as the
dialecticians say the Ideas are, there must be something much more
scientific than science-itself and something more mobile than
movement-itself; for these will be more of the nature of
actualities, while science-itself and movement-itself are potencies
for these.
Obviously, then, actuality is prior both to potency and to every
principle of change.
9

That the actuality is also better and more valuable than the
good potency is evident from the following argument. Everything of
which we say that it can do something, is alike capable of contraries,
e.g. that of which we say that it can be well is the same as that
which can be ill, and has both potencies at once; for the same potency
is a potency of health and illness, of rest and motion, of building
and throwing down, of being built and being thrown down. The
capacity for contraries, then, is present at the same time; but
contraries cannot be present at the same time, and the actualities
also cannot be present at the same time, e.g. health and illness.
Therefore, while the good must be one of them, the capacity is both
alike, or neither; the actuality, then, is better. Also in the case of
bad things the end or actuality must be worse than the potency; for
that which 'can' is both contraries alike. Clearly, then, the bad does
not exist apart from bad things; for the bad is in its nature
posterior to the potency. And therefore we may also say that in the

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