oof, or
disproof, impossible. Since all perceptions are equally true, the
child's perceptions must be just as much the truth as those of his
teachers. His teachers, {179} therefore, can teach him nothing. As to
discussion and proof, the very fact that two people dispute about
anything implies that they believe in the existence of an objective
truth. Their impressions, if they contradict each other, cannot both
be true. For if so, there is nothing to dispute about. Thus all proof
and refutation are rendered futile by the theory of Protagoras.

(4) If perception is truth, man is the measure of all things, in his
character as a percipient being. But since animals are also percipient
beings, the lowest brute must be, equally with man, the measure of all
things.

(5) The theory of Protagoras contradicts itself. For Protagoras admits
that what appears to me true is true. If, therefore, it appears to me
true that the doctrine of Protagoras is false, Protagoras himself must
admit that it is false.

(6) It destroys the objectivity of truth, and renders the distinction
between truth and falsehood wholly meaningless. The same thing is true
and false at the same time, true for you and false for me. Hence it
makes no difference at all whether we say that a proposition is true,
or whether we say that it is false. Both statements mean the same
thing, that is to say, neither of them means anything. To say that
whatever I perceive is true for me merely gives a new name to my
perception, but does not add any value to it.

(7) In all perception there are elements which are not contributed by
the senses. Suppose I say, "This piece of paper is white." This, we
might think, is a pure judgment of perception. Nothing is stated
except what I perceive by means of my senses. But on consideration it
turns out that this is not correct. First of all I must {180} think
"this piece of paper." Why do I call it paper? My doing so means that
I have classified it. I have mentally compared it with other pieces of
paper, and decided that it is of a class with them. My thought, then,
involves comparison and classification. The object is a compound
sensation of whiteness, squareness, etc. I can only recognise it as a
piece of paper by identifying these sensations, which I have now, with
sensations received from other similar objects in the past. And not
only must I recognize the sameness of the sensations, but I must
recognize their difference from other