The Complete Aristotle (eng.)

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.
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    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 things which are from the
beginning, i.e. in eternal things, there is nothing bad, nothing
defective, nothing perverted (for perversion is something bad).
    It is an activity also that geometrical constructions are
discovered; for we find them by dividing. If the figures had been
already divided, the constructions would have been obvious; but as
it is they are present only potentially. Why are the angles of the
triangle equal to two right angles? Because the angles about one
point are equal to two right angles. If, then, the line parallel to
the side had been already drawn upwards, the reason would have been
evident to any one as soon as he saw the figure. Why is the angle
in a semicircle in all cases a right angle? If three lines are
equal the two which form the base, and the perpendicular from the
centre-the conclusion is evident at a glance to one who knows the
former proposition. Obviously, therefore, the potentially existing
constructions are discovered by being brought to actuality; the
reason is that the geometer’s thinking is an actuality; so that the
potency proceeds from an actuality; and therefore it is by making
constructions that people come to know them (though the single
actuality is later in generation than the corresponding potency).
(See diagram.)
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    The terms ‘being’ and ‘non-being’ are employed firstly with
reference to the categories, and secondly with reference to the
potency or actuality of these or their non-potency or nonactuality,
and thirdly in the sense