He had the misfortune, common at that time, to offend the religious Athenians, {11} for he spoke slightingly of the gods, proposing to "banish their existence or non-existence from writing and speech." He was convicted of atheism, and his books were publicly burnt, and he himself, then seventy years of age, was either banished or at least was obliged to flee from Athens, and on his way to Sicily he lost his life in a shipwreck. Our knowledge of this book of Protagoras is due to the preservation of its argument by Plato in the dialogue "Theætetus." Protagoras, we are there told, taught that "man is the measure of all things--of the existence of things that are, and of the non-existence of things that are not." "You have read him?" asks Socrates, addressing Theætetus. "Oh yes, again and again," is Theætetus' reply. Plato was entirely opposed to the doctrine that Protagoras taught. It seemed to him to bring gods and men and tadpoles to one level as far as truth was concerned; for he drew the deduction that if man is the measure of all things, then to each man his own opinion is right. Plato opposed to it the theory that truth is the vision of a pure objective reality. This same problem that exercised the ancient world is now again a chief centre of philosophical interest, and the aim of this little book is not to decide that question, but to serve as a guide and introduction to those who desire to know what the question is that divides philosophers to-day into the hostile camps of pragmatism and intellectualism.