On The Soul
actually. The power of sense is parallel to what is combustible, for
that never ignites itself spontaneously, but requires an agent which
has the power of starting ignition; otherwise it could have set itself
on fire, and would not have needed actual fire to set it ablaze.
In reply we must recall that we use the word 'perceive' in two ways,
for we say (a) that what has the power to hear or see, 'sees' or
'hears', even though it is at the moment asleep, and also (b) that
what is actually seeing or hearing, 'sees' or 'hears'. Hence 'sense'
too must have two meanings, sense potential, and sense actual.
Similarly 'to be a sentient' means either (a) to have a certain
power or (b) to manifest a certain activity. To begin with, for a
time, let us speak as if there were no difference between (i) being
moved or affected, and (ii) being active, for movement is a kind of
activity-an imperfect kind, as has elsewhere been explained.
Everything that is acted upon or moved is acted upon by an agent which
is actually at work. Hence it is that in one sense, as has already
been stated, what acts and what is acted upon are like, in another
unlike, i.e. prior to and during the change the two factors are
unlike, after it like.
But we must now distinguish not only between what is potential and
what is actual but also different senses in which things can be said
to be potential or actual; up to now we have been speaking as if
each of these phrases had only one sense. We can speak of something as
'a knower' either (a) as when we say that man is a knower, meaning
that man falls within the class of beings that know or have knowledge,
or (b) as when we are speaking of a man who possesses a knowledge of
grammar; each of these is so called as having in him a certain
potentiality, but there is a difference between their respective
potentialities, the one (a) being a potential knower, because his kind
or matter is such and such, the other (b), because he can in the
absence of any external counteracting cause realize his knowledge in
actual knowing at will. This implies a third meaning of 'a knower'