And in general it is because these thinkers suppose knowledge to
be sensation, and this to be a physical alteration, that they say that
what appears to our senses must be true; for it is for these reasons
that both Empedocles and Democritus and, one may almost say, all the
others have fallen victims to opinions of this sort.
Metaphysics
by Aristotle
Written circa 350 B.C.
Translated by W. D. Ross
Book IV.5
Regarding the nature of truth, we must maintain that not
everything which appears is true; firstly, because even if
sensation-at least of the object peculiar to the sense in
question-is not false, still appearance is not the same as
sensation.-Again, it is fair to express surprise at our opponents'
raising the question whether magnitudes are as great, and colours
are of such a nature, as they appear to people at a distance, or as
they appear to those close at hand, and whether they are such as
they appear to the healthy or to the sick, and whether those things
are heavy which appear so to the weak or those which appear so to
the strong, and those things true which appear to the slee ing or to
the waking. For obviously they do not think these to be open
questions; no one, at least, if when he is in Libya he has fancied one
night that he is in Athens, starts for the concert hall.-And again
with regard to the future, as Plato says, surely the opinion of the
physician and that of the ignorant man are not equally weighty, for
instance, on the question whether a man will get well or not.-And
again, among sensations themselves the sensation of a foreign object
and that of the appropriate object, or that of a kindred object and
that of the object of the sense in question, are not equally
authoritative, but in the case of colour sight, not taste, has the
authority, and in the case of flavour taste, not sight; each of
which senses never says at the same time of the same object that it
simultaneously is 'so and not so'.-But not even at different times
does one sense disagree about the quality, but only about that to
which the quality belongs. I mean, for instance, that the same wine
might seem, if either it or one's body changed, at one time sweet
and at another time not sweet; but at least the sweet, such as it is
when it exists, has never yet changed, but one is always right about
it, and that which is to be sweet is of necessity of such and such a
nature. Yet all these views destroy this necessity, leaving nothing to
be of necessity, as they leave no essence of anything; for the
necessary cannot be in this way and also in that, so that if
anything is of necessity, it will not be 'both so and not so'.
And, in general, if only the sensible exists, there would be
nothing if animate things were not; for there would be no faculty of
sense. Now the view that neither the sensible qualities nor the
sensations would exist is doubtless true (for they are affections of
the perceiver), but that the substrata which cause the sensation
should not exist even apart from sensation is impossible. For
sensation is surely not the sensation of itself, but there is
something beyond the sensation, which must be prior to the
sensation; for that which moves is prior in nature to that which is
moved, and if they are correlative terms, this is no less the case.
Movement