Since that which is 'capable' is capable of something and at
some time in some way (with all the other qualifications which must be
present in the definition), and since some things can produce change
according to a rational formula and their potencies involve such a
formula, while other things are nonrational and their potencies are
non-rational, and the former potencies must be in a living thing,
while the latter can be both in the living and in the lifeless; as
regards potencies of the latter kind, when the agent and the patient
meet in the way appropriate to the potency in question, the one must
act and the other be acted on, but with the former kind of potency
this is not necessary. For the nonrational potencies are all
productive of one effect each, but the rational produce contrary
effects, so that if they produced their effects necessarily they would
produce contrary effects at the same time; but this is impossible.
There must, then, be something else that decides; I mean by this,
desire or will. For whichever of two things the animal desires
decisively, it will do, when it is present, and meets the passive
object, in the way appropriate to the potency in question. Therefore
everything which has a rational potency, when it desires that for
which it has a potency and in the circumstances in which it has the
potency, must do this. And it has the potency in question when the
passive object is present and is in a certain state; if not it will
not be able to act. (To add the qualification 'if nothing external
prevents it' is not further necessary; for it has the potency on the
terms on which this is a potency of acting, and it is this not in
all circumstances but on certain conditions, among which will be the
exclusion of external hindrances; for these are barred by some of
the positive qualifications.
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