On The Soul
sense' covers three kinds of objects, two kinds of which are, in our
language, directly perceptible, while the remaining one is only
incidentally perceptible. Of the first two kinds one (a) consists of
what is perceptible by a single sense, the other (b) of what is
perceptible by any and all of the senses. I call by the name of
special object of this or that sense that which cannot be perceived by
any other sense than that one and in respect of which no error is
possible; in this sense colour is the special object of sight, sound
of hearing, flavour of taste. Touch, indeed, discriminates more than
one set of different qualities. Each sense has one kind of object
which it discerns, and never errs in reporting that what is before
it is colour or sound (though it may err as to what it is that is
coloured or where that is, or what it is that is sounding or where
that is.) Such objects are what we propose to call the special objects
of this or that sense.
'Common sensibles' are movement, rest, number, figure, magnitude;
these are not peculiar to any one sense, but are common to all.
There are at any rate certain kinds of movement which are
perceptible both by touch and by sight.
We speak of an incidental object of sense where e.g. the white
object which we see is the son of Diares; here because 'being the
son of Diares' is incidental to the directly visible white patch we
speak of the son of Diares as being (incidentally) perceived or seen
by us. Because this is only incidentally an object of sense, it in
no way as such affects the senses. Of the two former kinds, both of
which are in their own nature perceptible by sense, the first
kind-that of special objects of the several senses-constitute the
objects of sense in the strictest sense of the term and it is to
them that in the nature of things the structure of each several
sense is adapted.