It seems certain, and would in any case be reasonably expected, that from the earliest days of Greek philosophy the inclination towards bold assertion was accompanied by a contrary tendency towards questioning, doubt, and some-times despair. Among the pre-Socratics Xenophanes and Empedocles expressed occasionally the gloomy feeling that, in the welter of conflicting doctrines, it was really impossible to find any assertion deserving of full belief; and doubtless many silent members of the philosophers' audiences listened with many reservations to what they were told. But eventually scepticism itself became a doctrine. Pyrrho of Ells, who died at nearly ninety years of age in about 275 B.C., had accompanied Alexander the Great on his expedition to India. Another philosopher present on that occasion was Anaxarchus, a disciple of Metrodorus of Chios, who was in turn an exponent of the theories of Democritus. By Democritus' followers, his view of the insufficiency of mere sense perception had been given a definitely sceptical emphasis, and it may thus be that Pyrrho derived from Anaxarchus the first impulse towards that general questioning and doubt for the propagation of which, almost paradoxically, he later established a school in his native city.
Scepticism as a philosophy shared with all other post-Aristotelian doctrines the characteristic of being directly practical in intention, recommended, like the rest, as a means of achieving non-attachment, thence peace of mind, and therefore such happiness as could be expected in a dangerous world. According to Pyrrho's pupil Timon of Phlius, the nature of things is completely unknowable. If so, the only proper attitude is that of reservation of judgment. Instead, for example, of worrying himself over questions of good and evil, a man should accept with a good grace law and tradition, and find tranquillity in cool conventionality of belief and conduct. It was commonly supposed that knowledge could be acquired by deductive argument. But a deductive argument must start from premises. If these premises are supposed to be known, they must themselves be conclusions in a course of argument which, in the end, can only be circular. And if they are not supposed to be known, then nothing deduced from them can be known either. We may sometimes permit ourselves a judgment of probabilities, but the pursuit of knowledge must simply be abandoned as vain.
It appears that in later life Timon lived at Athens; and while he was there the principles of Pyrrho became evidently the official doctrine of Plato's Academy. The Academy at this time, as Tarn points out, was preoccupied with resistance to Stoicism. Its leader, Arcesilaus of Pitane, who consistently with his principles published no writings, was mainly concerned to attack the admittedly dogmatic Stoic theory of knowledge. Whereas Zeno had vehemently insisted that some "impressions" are so forceful that they compel assent and leave no room for mistake, Arcesilaus asserted that any "impression" might be deceptive. Unquestioning conviction was always ill-founded. He appears to have believed that in taking this sceptical line he was being faithful at least to the early Platonic position, in which Socrates is represented as disclaiming knowledge for himself and calling in question all assertions put forward by others. But according to Cicero, he held that even Socrates was mistaken, in that he purported to know that nothing could be known with certainty. True scepticism, as the physician Sextus Empiricus expressed it much later, was like a purgative medicine, which carries away itself along with other substances.
Distinguished among later leaders of the Academy, and also a Sceptic, was Carneades, apparently a powerful and vociferous orator, whose readiness to argue with fervor on both sides of any question aroused much excitement and some censure when, in 156 B.C., he lectured in the stern, moralistic atmosphere of Rome. But by far the most extensive and systematic exposition of Sceptical arguments was made much later, at a time when the Academy had been supposedly purified by a return to the pure doctrine of Timon and Pyrrho — indeed revived (by Aenesidemus) after a period of near-extinction.
Sextus Empiricus, who wrote in the second half of the second century A.D., offered in three treatises a complete compendium of sceptical arguments against, in effect, all those who claimed definite knowledge in any field. He included in his attack not only, as one would expect, the philosophers of competing schools, but also mathematicians, such harmless persons as grammarians, and such relatively easy game as astrologers. His work is of some importance in the history of logic, and much of his argumentation is undeniably clever. The effect of his questioning is, however, fatally weakened by its undiscriminating and mechanical character. The reader soon comes to feel that he is resolved to question not only the genuinely questionable, but anything whatever, and that he is prepared to do this, if solid argument will not serve, by any logic-chopping device that will have the effect of producing at least temporary perplexity. There is something disagreeably prolix and pedantic in his incessant criticism, which has in the end the lack of interest of all purely destructive negation. In view of this it is hardly surprising that, although the "dogmatic" philosophers never answered the arguments marshaled by the Sceptics against them, yet these arguments seem to have made almost no effect. By most people the professional Sceptic seems never to have been taken seriously, and this is probably because it was felt that he was not fully in earnest. As Sextus himself insisted, "the Sceptic does not frame his life as a man according to the doctrine which he professes as a philosopher." And although the Sceptic would urge that this is equally true of, for instance, the Stoic, the Stoic paid to his doctrine the tribute at least of trying to live as that doctrine prescribed. The tranquillity of mind which it was the aim of Scepticism to provide seemed too empty an affair to tempt many adherents.
One distinguished late disciple of the Sceptical doctrine was Lucian of Samosata, whose dialogue Hermotimus shows an affable Sceptic triumphing over an earnest and laborious student of Stoicism (though most of the arguments are applicable to any fixed doctrine whatever). This is, by contrast with the Aridities of Sextus, an unpedantic and even entertaining piece, in which the excesses of solemn philosophy are mildly ridiculed from the standpoint of a critical, detached, easy-minded man of the world. Almost in the spirit of Gibbon, or Hume, or the Earl of Chesterfield, he advocates as suitable for a reasonable man an acceptance of the ordinary ways of the world, since the rivalries of philosophers serve only to make their devotees ill-tempered, unreasonably dogmatic, and ridiculous.