Even if perception as a faculty is of 'the such' and not
merely of a 'this somewhat', yet one must at any rate actually
perceive a 'this somewhat', and at a definite present place and
time: but that which is commensurately universal and true in all cases
one cannot perceive, since it is not 'this' and it is not 'now'; if it
were, it would not be commensurately universal-the term we apply to
what is always and everywhere. Seeing, therefore, that
demonstrations are commensurately universal and universals
imperceptible, we clearly cannot obtain scientific knowledge by the
act of perception: nay, it is obvious that even if it were possible to
perceive that a triangle has its angles equal to two right angles,
we should still be looking for a demonstration-we should not (as
some say) possess knowledge of it; for perception must be of a
particular, whereas scientific knowledge involves the recognition of
the commensurate universal. So if we were on the moon, and saw the
earth shutting out the sun's light, we should not know the cause of
the eclipse: we should perceive the present fact of the eclipse, but
not the reasoned fact at all, since the act of perception is not of
the commensurate universal.
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