For nature also is in the same
genus as potency; for it is a principle of movement-not, however, in
something else but in the thing itself qua itself. To all such
potency, then, actuality is prior both in formula and in
substantiality; and in time it is prior in one sense, and in another
not.
(1) Clearly it is prior in formula; for that which is in the
primary sense potential is potential because it is possible for it
to become active; e.g. I mean by 'capable of building' that which
can build, and by 'capable of seeing' that which can see, and by
'visible' that which can be seen. And the same account applies to
all other cases, so that the formula and the knowledge of the one must
precede the knowledge of the other.
(2) In time it is prior in this sense: the actual which is
identical in species though not in number with a potentially
existing thing is to it. I mean that to this particular man who now
exists actually and to the corn and to the seeing subject the matter
and the seed and that which is capable of seeing, which are
potentially a man and corn and seeing, but not yet actually so, are
prior in time; but prior in time to these are other actually
existing things, from which they were produced. For from the
potentially existing the actually existing is always produced by an
actually existing thing, e.g. man from man, musician by musician;
there is always a first mover, and the mover already exists
actually. We have said in our account of substance that everything
that is produced is something produced from something and by
something, and that the same in species as it.
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