The Complete Aristotle (eng.)

capable of
building, and the waking to the 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.
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7
    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 which comes to be, a thing is
potentially all those things which it will be of itself if nothing
external hinders it. E.g. the seed is not yet potentially a man;
for it must be deposited in something other than itself and undergo
a change. But when through its own motive principle it has already
got such