Protagoras's works include On God, On Truth and On Paradox. The first sentence at the beginning of On God is: "As for God, I don't know that they exist, and I don't know that they don't exist. There are many things that hinder our understanding, such as obscure problems and short life. " The most famous philosophical quote he left behind is in On Truth: "Man is the measure of everything, everything exists when it exists, and it does not exist when it does not exist." His words angered the Athenians. His works were collected from plagiarists and burned in public in the square. He himself was expelled from the polis. His main ideas were recorded through some dialogues of Plato later.