Protagoras was famous as a teacher of rhetoric and debate which were vital to Greek social life. Due to those interests, he was fascinated by the study of orthoepeia, or the correct use of words.

His most famous saying is: "Man is the measure of all things: of things which are, that they are so, and of things which are not, that they are not."1 The word 'man' here is used generically meaning any human being. A subjectivist approach would see this as an individual, but it is perhaps more likely that Protagoras came from a relativist angle and meant humans collectively.

Despite the fame of this phrase, it has been passed down to us without any context, as is so often the case with the Presocratics, and its meaning isn't entirely clear. It was Protagoras' teachings that spurred later philosophers such as Plato to search for objective, transcendent guidelines to underlie moral behavior, and the importance of subjectivity is an important theme in modern philosophy.

Protagoras was also a famous proponent of agnosticism. In "On the Gods," he wrote, "Concerning the gods, I have no means of knowing whether they exist or not or of what sort they may be, because of the obscurity of the subject, and the brevity of human life."

The Protagoras crater on the Moon was named in his honor.

Protagoras and the scientific method
Even though Protagoras was a contemporary of Socrates, the philosopher of Abdera is considered a presocratic thinker. He followed the Ionian tradition that distinguishes the School of Abdera. The distinctive note of this tradition is criticism, a systematic discussion that can be identified as "presocratic dialectic" which was an alternative to the aristotelian demonstrative method which, according to Karl Popper, has the fault of dogmatism.

Maybe the main contribution of Protagoras was in the field of Epistemology due to his method to find a better argument by discarding the less viable one. It is known as "Antilogies" consisting of two premises. The first one was "Before any uncertainty two opposite theses can validly be confronted". And the second is its complement: the need to "strengthen the weakest argument".

Protagoras knew that the less appealing argument could hide the best answer, which is why he stated that it was constantly necessary to strengthen the weakest argument. Having been born before Socrates himself, this progressive viewpoint in the development of consensual truth could conceivably have contributed to the progressive styles of many of the other great minds which followed him.

Protagoras is also a dialogue by Plato.