To deny the distinction between objective truth and subjective impression is the same as to deny the distinction between reason and sense. To my senses the earth seems flat. It looks flat to the eye. It is only through reason that I know the objective truth that the world is round. Reason, therefore, is the only possible standard of objective truth. If you deny the rational element its proper part, it follows that you will be left a helpless prey to diverse personal impressions. The impressions yielded by the senses differ in different people. One man sees a thing in one way, another sees it in another. If, therefore, what seems to me true is true for me, and what seems to you true is true for you, and if our impressions differ, it will follow that two contradictory propositions must both be true. Protagoras clearly understood this, {116} and did not flinch from the conclusion. He taught that all opinions are true, that error is impossible, and that, whatever proposition is put forward, it is always possible to oppose to it a contradictory proposition with equally good arguments and with equal truth. In reality, the result of this procedure is to rob the distinction between truth and falsehood of all meaning. It makes no difference whether we say that all opinions are true, or whether we say that all are false. The words truth and falsehood, in such context, have no meaning. To say that whatever I feel is the truth for me means only that what I feel I feel. To call this "truth for me," adds nothing to the meaning. Protagoras seems to have been led to these doctrines partly by observing the different accounts of the same object which the sense-organs yield to different people, and even to the same person at different times. If knowledge depends upon these impressions, the truth about the object cannot be ascertained. He was also influenced by the teaching of Heracleitus. Heracleitus had taught that all permanence is illusion. Everything is a perpetual becoming; all things flow. What is at this moment, at the next moment is not. Even at one and the same moment, Heracleitus believed, a thing is and is not. If it is true to say that it is, it is equally true that it is not. And this is, in effect, the teaching of Protagoras. The Protagorean philosophy thus amounts to a declaration that knowledge is impossible. If there is no objective truth, there cannot be any knowledge of it. The impossibility of knowledge is also the standpoint of Gorgias. The title of his book is characteristic of {117} the Sophistical love of paradox. It was called "On Nature, or the non-existent." In this book he attempted to prove three propositions, (1) that nothing exists: (2) that if anything exists, it cannot be known: (3) that if it can be known, the knowledge of it cannot be communicated.