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.