In addition to their activities as teachers and statesmen, the Sophists wrote treatises on a staggering array of subjects, including rhetoric, debate, poetry, music, natural science, geometry, theology, and government. The surviving fragments of these treatises suggest that the Sophists' philosophical interests and doctrines varied widely. (One of the most common mistakes has been to treat the Sophists as a unified movement or school.) Protagoras is best known for the opening (and only surviving) lines of two works. His book On Truth began: "Man is the measure of all things, of things that are that they are, and of things that are not that they are not." Debate has centered on whether this fragment represents some form of secular humanism (i.e., man, as opposed to the gods, measures the truth of all things), or a type of subjectivist relativism (i.e., each man measures what is true for himself). The latter interpretation is most plausible, though the former is sometimes supported by reference to the famous fragment from Protagoras's work, On the Gods: "Concerning the gods I cannot know either that they exist or that they do not exist, or what form they might have, for there is much to prevent one's knowing: the obscurity of the subject and the shortness of man's life."
The Sophist Prodicus also wrote on the gods, arguing anthropologically that what are called gods are "things useful for human life." Whether his theory was meant to support atheism or rather to bolster belief in the gods by relating them to human needs is not clear. Prodicus was more famous in antiquity for his art of defining words (called "synonymic by German scholars), which enabled his students to untangle vexing paradoxes by distinguishing precisely between terms and concepts.
The fragments of Hippias of Elis testify to the breath of learning of the Sophists. If his work, The Collection (Synagogê), was an encyclopedia of some kind, which is likely, it would help explain why he is cited well into the Middle Ages on a host of diverse topics—from the etymology of particular words, to the biographical details of famous personages, to questions of astronomy and geometry. Perhaps the most familiar Sophistic doctrine today is the one put forth by Thrasymachus in book one of Plato's Republic, namely that "justice is the advantage of the stronger." It is not certain that Thrasymachus should be grouped among the Sophists (Plato himself does not do so), but his oft-quoted maxim has been used to give the Sophists a bad name. It is interesting to compare Thrasymachus' views to those of the Anonymous Iamblichi, a short treatise thought to be composed by a Sophist of the late fifth-century B.C.E., in which laws are similarly understood as merely conventional and yet defended as the source of political stability.