The first major thinker to move to Athens from abroad was Anaxagoras. Arriving in 464 BC, he became a friend of Pericles and outraged religious opinion by claiming (correctly) that the moon was lit by reflected sunlight.

He was followed nearly 20 years later by Protagoras, who also became an associate of Pericles, as well as the leading figure of a loose collection of thinkers called (by their opponents) the Sophists.

Arguing that 'man is the measure of all things', the Sophists were skeptical about the existence of the gods and taught a variety of subjects, including mathematics, grammar, physics, political philosophy, ancient history, music, and astronomy. However, they were best known for teaching rhetoric, the skill of arguing with conviction. Because of this their talents were much in demand by aspiring politicians.

The Sophists did not all believe or follow the same things. For instance, some Sophists believed in democracy, while others argued that 'might is right' and advocated rule by oligarchies and tyrants. Most of what we know of the Sophists comes from Socrates' pupil, Plato, who portrays them in a very negative light because they accepted fees and claimed that virtues could be taught.

Nevertheless some of the Sophists, like Protagoras, were very idealistic. During the Peloponnesian War, he defied new laws restricting what could be taught and as a result many of his books were destroyed in the first known book burning. Sent into exile, he was lost at sea in 411 BC.