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.
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