The Complete Aristotle (eng.)

make the opposed affirmations and negations
truly of the same subject. Further, if the affirmation is no more
true than the negation, he who says ‘man’ will be no more right
than he who says ‘not-man’. It would seem also that in saying the
man is not a horse one would be either more or not less right than
in saying he is not a man, so that one will also be right in saying
that the same person is a horse; for it was assumed to be possible
to make opposite statements equally truly. It follows then that the
same person is a man and a horse, or any other animal.
    While, then, there is no proof of these things in the full
sense, there is a proof which may suffice against one who will make
these suppositions. And perhaps if one had questioned Heraclitus
himself in this way one might have forced him to confess that
opposite statements can never be true of the same subjects. But, as
it is, he adopted this opinion without understanding what his
statement involves. But in any case if what is said by him is true,
not even this itself will be true-viz. that the same thing can at
one and the same time both be and not be. For as, when the
statements are separated, the affirmation is no more true than the
negation, in the same way-the combined and complex statement being
like a single affirmation-the whole taken as an affirmation will be
no more true than the negation. Further, if it is not possible to
affirm anything truly, this itself will be false-the assertion that
there is no true affirmation. But if a true affirmation exists,
this appears to refute what is said by those who raise such
objections and utterly destroy rational discourse.
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6
    The saying of Protagoras is like the views we have mentioned; he
said that man is the measure of all things, meaning simply that
that which seems to each man also assuredly is. If this is so, it
follows that the same thing both is and is not, and is bad and
good, and that the contents of all other opposite statements are
true, because often a particular thing appears beautiful to some
and the contrary of beautiful to others, and that which appears to
each man is the measure. This difficulty may be solved by
considering the source of this opinion. It seems to have arisen in
some cases from the doctrine of the natural philosophers, and in
others from the fact that all men have not the same views about the
same things, but a particular thing appears pleasant to some and
the contrary of pleasant to others.
    That nothing comes to be out of that which is not, but
everything out of that which is, is a dogma common to nearly all
the natural philosophers. Since, then, white cannot come to be if
the perfectly white and in no respect not-white existed before,
that which becomes white must come from that which is not white; so
that it must come to be out of that which is not (so they argue),
unless the same thing was at the beginning white and not-white. But
it is not hard to solve this difficulty; for we have said in our
works on physics in what sense things that come to be come to be
from that which is not, and in what sense from that which is.
    But to attend equally to the opinions and the fancies of
disputing parties is childish; for clearly one of them must be
mistaken. And this is evident from what happens in respect of
sensation; for the same thing never appears sweet to some and the
contrary of sweet to others, unless in the one case the sense-organ
which discriminates the aforesaid flavours has been perverted and
injured. And if this is so the one party must be taken to be the
measure, and the other must not. And say the same of good and bad,
and beautiful and ugly, and all other such qualities. For to
maintain the view we are opposing is just like maintaining that the
things that appear to people who put their finger under their eye
and make the object appear two instead of one must be two (because
they appear to be of that number) and again one (for to those who
do not interfere with their eye the one object appears one).
    In general, it is absurd to make the fact that the things of
this earth are observed to change and never to remain in the same
state, the basis of our judgement about the truth. For in pursuing
the truth one must start from the things that are always in the
same state and suffer no change. Such are the heavenly bodies; for
these do not appear to be now of one nature and again of another,
but are manifestly always the