Protagoras

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 orthopaedia, or the correct use of words.

His most famous saying is: "Man is the measure of all things, of those that are that they are, and of those that are not that they are not." Despite its fame, this phrase was passed down absent any context, and its meaning isn't entirely clear. It is generally thought to be promoting relativism. It was his teachings that spurred later philosophers such as Plato to search for objective, transcendent guidelines to underly 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."