Critolaus (/kraɪtoʊˈleɪəs/; Greek: Κριτόλαος Kritolaos; c. 200-c. 118 BC) of Phaselis was a Greek philosopher of the Peripatetic school. He was one of three philosophers sent to Rome in 155 BC (the other two being Carneades and Diogenes of Babylon), where their doctrines fascinated the citizens, but scared the more conservative statesmen. None of his writings survive. He was interested in rhetoric and ethics, and considered pleasure to be an evil. He maintained the Aristotelian doctrine of the eternity of the world, and of the human race in general, directing his arguments against the Stoics.
He was born in Phaselis, a Greek colony in Lycia, c. 200 BC, and studied philosophy at Athens under Aristo of Ceos, and became one of the leaders of the Peripatetic school by his eminence as an orator, a scholar and a moralist. There has been considerable discussion as to whether he was the immediate successor of Aristo, but the evidence is confused.
The great reputation which Critolaus enjoyed at Athens, as a philosopher, an orator, and a statesman, induced the Athenians to send him to Rome in 155 BC, together with Carneades and Diogenes the Stoic, to obtain a remission of the fine of 500 talents which the Romans had imposed upon Athens for the destruction of Oropus. They were successful in the object for which they came; and the embassy excited the greatest interest at Rome. Not only the Roman youth, but the most illustrious men in the state, such as Scipio Africanus, Laelius, Furius, and others, came to listen to their discourses. The novelty of their doctrines seemed to the Romans of the old school to be fraught with such danger to the morals of the citizens, that Cato induced the senate to send them away from Rome as quickly as possible. Gellius describes his arguments as "elegant and polished" (Latin: scita et teretia). We have no further information respecting the life of Critolaus. He lived upwards of eighty-two years, but died c. 118 BC. By the time Licinius Crassus arrived at Athens c. 111 BC, he found Critolaus' pupil Diodorus of Tyre at the head of the Peripatetic school.