them, and
only reason is aware of their existence. Now the teaching of
Protagoras really rests fundamentally upon the denying and confusing
of this distinction. If we are to see this, we must first of all
understand that reason is the universal, sensation the particular,
element in man. In the first place, reason is communicable, sensation
incommunicable. My sensations and feelings are personal to myself, and
cannot be imparted to other people. For example, no one can
communicate the sensation of redness to a colour-blind man, who has
not already experienced it. But a thought, or rational idea, can be
communicated to any rational being. Now suppose the question is
whether the angles at the base of an isosceles triangle are equal. We
may approach the problem in two ways. We may appeal either to the
senses or to reason. If we appeal to the senses, one man will come
forward and say that to him the angles look equal. Another man will
say that one angle looks bigger than the other, and so on. But if,
like Euclid, we appeal to reason, then it can be proved that the two
angles are equal, and there is no room left for mere personal
impressions, because reason is a law universally valid and binding
upon all men. My sensations are private and peculiar to myself. They
bind no one but myself. My {114} impressions about the triangle are
not a law to anyone except myself. But my reason I share with all
other rational beings. It is not a law for me merely, but for all. It
is one and the same reason in me and in other men. Reason, therefore,
is the universal, sensation the particular, element in man. Now it is
practically this distinction that Protagoras denied. Man, he said, is
the measure of all things. By man he did not mean mankind at large. He
meant the individual man. And by measure of all things he meant the
standard of the truth of all things. Each individual man is the
standard of what is true to himself. There is no truth except the
sensations and impressions of each man. What seems true to me is true
for me. What seems true to you is true for you.

We commonly distinguish between subjective impressions and objective
truth. The words subjective and objective are constantly recurring
throughout the history of philosophy, and as this is the first time I
use them, I will explain them here. In every act of thought there must
necessarily be two terms. I am now looking at this desk and thinking
of this desk. There is the "I" which