Carneades (ca. 213-ca. 128 BC) was a Greek philosopher of the third school of academic skepticism. His combination of skepticism and empiricism can now be seen to have remarkable affinities with a good deal of post-Renaissance Western philosophy.

Carneades was born in Cyrene. Little is known of his personal life, except that in 156 B.C. he came to Rome, along with two other philosophers, to protest a recent fine imposed on Athens by Rome. Here he demonstrated with great effect the logic of skepticism by delivering two contradictory orations on justice. One of these praised justice, as a virtue grounded in nature; the other praised injustice, on grounds of expediency.

Carneades spent a large part of his long life as head of the so-called Third Academy, which like the second, started by Arcesilaus, was grounded in skepticism. In his attacks on "dogmatic" philosophies, particularly the stoicism of Chrysippus, Carneades went far beyond the skepticism of Arcesilaus. Accepting Arcesilaus's contention against the Stoics that "presentations" are just as likely to be untrue as true, he undermined the doctrine further by suggesting, in anticipation of George Berkeley, that at least one thing, color, has no absolute presentation at all but varies according to time and circumstance. He attacked the whole notion of a law of contradiction and may have been the first to suggest that the notion of "proof" in philosophy is chimerical.

In contrast with this, Carneades seems to have been more moderate than other Skeptics in his suggestion that some presentations are less misleading than others. He ridiculed the Stoics' belief in the gods, in providence, and in divine creation, and he undermined their belief in man's ability to predict the future by means of omens, dreams, oracles, and the like, by arguing that reference to chance alone is enough to account for any of their supposed successes. The determinism of stoicism he attempted to subvert by suggesting that man's free will is an independent cause, outside the scheme of physical cause-effect relationships.

Carneades suggested--in contrast with Chrysippus--that there is no such thing as a law grounded in nature, and he anticipated Thomas Hobbes in arguing that man is not just by nature. Skeptical of the whole notion of moral absolutes, he propounded a "social contract" theory of justice that has a remarkably modern ring, and his analysis of justice in terms of utility seems to antedate Jeremy Bentham and John Stuart Mill.