Skepticism on philosophy has its roots in ancient Greek philosophy. The Greek Sophists of the 5th century BC, such as Protagoras of Abdera (480–411 BC), were mostly skeptics. Gorgias (485–380 BC) said, "Nothing exists; if anything does exist, it cannot be known; if anything exists and can be known, it cannot be communicated". In the 4th century BC Pyrrho of Elis (c360–275 BC), who travelled and studied as far as India, adopted practical skepticism. Carneades (c213–129 BC) disagreed that things were completely true or false. He criticized the Dogmatists, especially Stoics. He said that completely certain knowledge is impossible. Sextus Empiricus (c200 AD), the main authority for Greek skepticism, brought together empiricism into the basis for stating knowledge.