A further possibility is that these things should be classified
according to their effects, and that it is reasonable to do so with
invisible entities, such as sciences; but this would not be applicable
to sense-objects. When indeed we divide sciences by their effects, and
when, in general, we classify them according to the powers of the soul,
by concluding from the diversity of their effects that they differ,
our mind grasps the difference of these powers, and it determines not
only with what objects they deal, but it also defines their reason (or,
essence). Let us admit that it is easy to distinguish arts according
to their reasons, and according to the notions they include; but is it
possible to divide corporeal qualities in that manner? Even when one
studies the intelligible world, there is room for doubt as to how the
different reasons distinguish themselves from each other; it is easy
enough to see that white differs from black; but in what does it do so?
IT IS ABSURD TO DISTINGUISH BEING, QUALITIES AND DIFFERENCES BY
THEMSELVES.
18. All the questions we have asked show that we doubtless must
seek to discover the differences of the various (beings), so as to
distinguish them from each other; but that it is as impossible as it
is unreasonable to inquire what are the differences of the differences
themselves.[408] Being of beings, quantities of quantities, qualities
of qualities, differences of differences cannot be discovered; but we
should, wherever possible, classify exterior objects, either according
to their effects, or according to salient characteristics. When this is
impossible, objects should be distinguished, as for instance dark from
light green.
But how is white distinguished from black? Sensation or intelligence
tell us that those things are different without informing us of their
reason; either sensation, because its function is not to set forth the
reason of things, but only to bring them somehow to our attention; or
intelligence, because it discerns things that are simple by intuition,
without having to resort to ratiocination, and limits itself to the
statement that something is such or such. Besides, in each one of the
operations of intelligence there is a difference (a special distinctive
characteristic) which enables it to distinguish different things,
without this difference (which is proper to each of the operations of
intelligence) itself having need to be discerned by the help of some
other difference.