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sleeping, and that which is seeing to that which has its eyes shut but
has sight, and that which has been shaped out of the matter to the
matter, and that which has been wrought up to the unwrought. Let
actuality be defined by one member of this antithesis, and the
potential by the other. But all things are not said in the same
sense to exist actually, but only by analogy-as A is in B or to B, C
is in D or to D; for some are as movement to potency, and the others
as substance to some sort of matter.
But also the infinite and the void and all similar things are said
to exist potentially and actually in a different sense from that which
applies to many other things, e.g. to that which sees or walks or is
seen. For of the latter class these predicates can at some time be
also truly asserted without qualification; for the seen is so called
sometimes because it is being seen, sometimes because it is capable of
being seen. But the infinite does not exist potentially in the sense
that it will ever actually have separate existence; it exists
potentially only for knowledge. For the fact that the process of
dividing never comes to an end ensures that this activity exists
potentially, but not that the infinite exists separately.
Since of the actions which have a limit none is an end but all are
relative to the end, e.g. the removing of fat, or fat-removal, and the
bodily parts themselves when one is making them thin are in movement
in this way (i.e. without being already that at which the movement
aims), this is not an action or at least not a complete one (for it is
not an end); but that movement in which the end is present is an
action. E.g. at the same time we are seeing and have seen, are
understanding and have understood, are thinking and have thought
(while it is not true that at the same time we are learning and have
learnt, or are being cured and have been cured). At the same time we
are living well and have lived well, and are happy and have been
happy. If not, the process would have had sometime to cease, as the
process of making thin ceases: but, as things are, it does not
cease; we are living and have lived. Of these processes, then, we must
call the one set movements, and the other actualities. For every
movement is incomplete-making thin, learning, walking, building; these
are movements, and incomplete at that. For it is not true that at
the same time a thing is walking and has walked, or is building and
has built, or is coming to be and has come to be, or is being moved
and has been moved, but what is being moved is different from what has
been moved, and what is moving from what has moved. But it is the same
thing that at the same time has seen and is seeing, seeing, or is
thinking and has thought. The latter sort of process, then, I call
an actuality, and the former a movement.

What, and what kind of thing, the actual is, may be taken as
explained by these and similar considerations. But we must distinguish
when a thing exists potentially and when it does not; for it is not at
any and every time. E.g. is earth potentially a man? No-but rather
when it has already become seed, and perhaps not even then. It is just
as it is with being healed; not everything can be healed by the
medical art or by luck, but there is a certain kind of thing which
is capable of it, and only this is potentially healthy. And (1) the
delimiting mark of that which as a result of thought comes to exist in
complete reality from having existed potentially is that if the
agent has willed it it comes to pass if nothing external hinders,
while the condition on the other side-viz. in that which is
healed-is that nothing in it hinders the result. It is on similar
terms that we have what is potentially a house; if nothing in the
thing acted on-i.e. in the matter-prevents it from becoming a house,
and if there is nothing which must be added or taken away or
changed, this is potentially a house; and the same is true of all
other things the source of whose becoming is external. And (2) in
the cases in which the source of the becoming is in the very thing

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