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.
For proof of the first proposition, "nothing exists," Gorgias attached himself to the school of the Eleatics, especially to Zeno. Zeno had taught that in all multiplicity and motion, that is to say, in all existence, there are irreconcilable contradictions. Zeno was in no sense a sceptic. He did not seek for contradictions in things for the sake of the contradictions, but in order to support the positive thesis of Parmenides, that only being is, and that becoming is not at all. Zeno, therefore, is to be regarded as a constructive, and not merely as a destructive, thinker. But it is obvious that by emphasizing only the negative element in his philosophy, it is possible to use his antinomies as powerful weapons in the cause of scepticism and nihilism. And it was in this way that Gorgias made use of the dialectic of Zeno. Since all existence is self-contradictory, it follows that nothing exists. He also made use of the famous argument of Parmenides regarding the origin of being. If anything is, said Gorgias, it must have had a beginning. Its being must have arisen either from being, or from not-being. If it arose from being, there is no beginning. If it arose from not-being, this is impossible, since something cannot arise out of nothing. Therefore nothing exists.
The second proposition of Gorgias, that if anything exists it cannot be known, is part and parcel of the whole Sophistic tendency of thought, which identifies knowledge [{118}] with sense-perception, and ignores the rational element. Since sense-impressions differ in different people, and even in the same person, the object as it is in itself cannot be known. The third proposition follows from the same identification of knowledge with sensation, since sensation is what cannot be communicated.
The later Sophists went much further than Protagoras and Gorgias. It was their work to apply the teaching of Protagoras to the spheres of politics and morals. If there is no objective truth, and if what seems true to each individual is for him the truth, so also, there can be no objective moral code, and what seems right to each man is right for him. If we are to have anything worth calling morality, it is clear that it must be a law for all, and not merely a law for some. It must be valid for, and binding upon, all men. It must, therefore, be founded upon that which is universal in man, that is to say, his reason. To found it upon sense-impressions and feelings is to found it upon shifting quicksands. My feelings and sensations are binding upon no man but myself, and therefore a universally valid law cannot be founded upon them. Yet the Sophists identified morality with the feelings of the individual. Whatever I think right is right for me. Whatever you think right is right for you. Whatever each man, in his irrational self-will, chooses to do, that is, for him, legitimate. These conclusions were drawn by Polus, Thrasymachus, and Critias.
Now if there is, in this way, no such thing as objective right, it follows that the laws of the State can be founded upon nothing except force, custom, and convention. We often speak of just laws, and good laws. But to speak in that way involves the existence of an objective [{119}] standard of goodness and justice, with which we can compare the law, and see whether it agrees with that standard or not. To the Sophists, who denied any such standard, it was mere nonsense to speak of just and good laws. No law is in itself good or just, because there is no such thing as goodness or justice. Or if they used such a word as justice, they defined it as meaning the right of the stronger; or the right of the majority. Polus and Thrasymachus, consequently, drew the conclusion that the laws of the State were inventions of the weak, who were cunning enough, by means of this stratagem, to control the strong, and rob them of the natural fruits of their strength. The law of force is the only law which nature recognizes. If a man, therefore, is powerful enough to defy the law with impunity, he has a perfect right to do so. The Sophists were thus the first, but not the last, to preach the doctrine that might is right. And, in similar vein, Critias explained popular belief in the gods as the invention of some crafty statesman for controlling the mob through fear.
Now it is obvious that the whole tendency of this sophistical teaching is destructive and anti-social. It is destructive of religion, of morality, of the foundations of the State, and of all established institutions. And we can now see that the doctrines of the Sophists were, in fact, simply the crystallization into abstract thought of the practical tendencies of the age. The people in practice, the Sophists in theory, decried and trod under foot the restrictions of law, authority, and custom, leaving nothing but the deification of the individual in his crude self-will and egotism. It was in fact an age of "aufklärung," which means enlightenment or [{120}] illumination. Such periods of illumination, it seems, recur periodically in the history of thought, and in the history of civilization. This is the first, but not the last, such period with which the history of philosophy deals. This is the Greek illumination. Such periods present certain characteristic features. They follow, as a rule, upon an era of constructive thought. In the present instance the Greek illumination followed closely upon the heels of the great development of science and philosophy from Thales to Anaxagoras. In such a constructive period the great thinkers bring to birth new principles, which, in the course of time, filter down to the masses of the people and cause popular, if shallow, science, and a wide-spread culture. Popular education becomes a feature of the time. The new ideas, fermenting among the people, break up old prejudices and established ideas, and thus thought, at first constructive, becomes, among the masses, destructive in character. Hence the popular thought, in a period of enlightenment, issues in denial, scepticism, and disbelief. It is merely negative in its activities and results. Authority, tradition, and custom are wholly or partially destroyed. And since authority, tradition, and custom are the cement of the social structure, there results a general dissolution of that structure into its component individuals. All emphasis is now laid on the individual. Thought becomes egocentric. Individualism is the dominant note. Extreme subjectivity is the principle of the age. All these features make their appearance in the Greek aufklärung. The Sophistical doctrine that the truth is what I think, the good what I choose to do, is the extreme application of the subjective and egocentric principles.
The early eighteenth century in England and France was likewise a period of enlightenment, and the era from which we are now, perhaps, just emerging, bears many of the characteristics of aufklärung. It is sceptical and destructive. All established institutions, marriage, the family, the state, the law, come in for much destructive criticism. It followed immediately upon the close of a great period of constructive thought, the scientific development of the nineteenth century. And lastly, the age has produced its own Protagorean philosophy, which it calls pragmatism. If pragmatism is not egocentric, it is at least anthropocentric. Truth is no longer thought of as an objective reality, to which mankind must conform. On the contrary, the truth must conform itself to mankind. Whatever it is useful to believe, whatever belief "works" in practice, is declared to be true. But since what "works" in one age and country does not "work" in another, since what it is useful to believe to-day will be useless to-morrow, it follows that there is no objective truth independent of mankind at all. Truth is not now defined as dependent on the sensations of man, as it was with Protagoras, but as dependent on the volition of man. In either case it is not the universal in man, his reason, which is made the basis of truth and morals, but the subjective, individual, particular element in him.
We must not forget the many merits of the Sophists. Individually, they were often estimable men. Nothing is known against the character of Protagoras, and Prodicus was proverbial for his wisdom and the genuine probity and uprightness of his principles. Moreover the Sophists contributed much to the advance of learning. [{122}] They were the first to direct attention to the study of words, sentences, style, prosody, and rhythm. They were the founders of the science of rhetoric. They spread education and culture far and wide in Greece, they gave a great impulse to the study of ethical ideas, which made possible the teaching of Socrates, and they stirred up a ferment of ideas without which the great period of Plato and Aristotle could never have seen the light. But, from the philosophical point of view, their merit is for the first time to have brought into general recognition the right of the subject. For there is, after all, much reason in these attacks made by the Sophists upon authority, upon established things, upon tradition, custom and dogma. Man, as a rational being, ought not to be tyrannized over by authority, dogma, and tradition. He cannot be subjected, thus violently, to the imposition of beliefs from an external source. No man has the right to say to me, "you shall think this," or "you shall think that." I, as a rational being, have the right to use my reason, and judge for myself. If a man would convince me, he must not appeal to force, but to reason. In doing so, he is not imposing his opinions externally upon me; he is educing his opinions from the internal sources of my own thought; he is showing me that his opinions are in reality my own opinions, if I only knew it. But the mistake of the Sophists was that, in thus recognizing the right of the subject, they wholly ignored and forgot the right of the object. For the truth has objective existence, and is what it is, whether I think it or not. Their mistake was that though they rightly saw that for truth and morality to be valid for me, they must be assented to by, and developed out of, [{123}] me myself, not imposed from the outside, yet they laid the emphasis on my merely accidental and particular characteristics, my impulses, feelings, and sensations, and made these the source of truth and morality, instead of emphasizing as the source of truth and right the universal part of me, my reason. "Man is the measure of all things"; certainly, but man as a rational being, not man as a bundle of particular sensations, subjective impressions, impulses, irrational prejudices, self-will, mere eccentricities, oddities, foibles, and fancies.