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The chief source for ancient Pyrrhonism is the work of the Greek physician Sextus Empiricus (second century C.E.), including Outlines of Pyrrhonism, Against the Dogmatists, and Against the Mathematicians. Once thought of as a mere compiler, many recent studies have found philosophical originality in his texts. As Sextus explained it, skepticism was not a philosophy but rather a way of life in which one opposed all claims to truth with equal opposite claims (equipollence). Standard tropes or formula arguments could be used against any certainty or truth. He attributed one set of these tropes to the Greek philosopher Aenesidemus and another to Agrippa (both first century B.C.E.). Diogenes Laertius's Lives of the Philosophers (early third century C.E.) is also a source for information about ancient skepticism, including the tropes.

In Sextus's account, the basic ten tropes or formula arguments show that the same thing appears differently (1) to different animals, (2) to different individuals, (3) to different senses, (4) to the same sense in different conditions, (5) in different positions or places, (6) in company with different things, (7) in different quantities, (8) in different relations, (9) if common or if rare, and (10) to people with different customs or ways of life. Thus, any claim about a thing could be matched with an equal counterclaim. Other tropes bring out the problem of the criterion (an infinite regress), unresolved disputes, problems with attributing causation, and more. The result of the skeptical tropes was that one would suspend judgment (epochē) and then find oneself in ataraxia, or tranquility, no longer disturbed by conflicting claims. One would live in accordance with the phenomena or appearances, without taking a stand on the truth or reality behind them. One would follow one's natural impulses as well as local customs and laws.

Even in ancient times, critics of the skeptics accused them of inconsistency, incoherence, immorality, and inability to live their skepticism. These arguments were more and less sophisticated, and ranged widely from the claim that skeptics cannot be fully skeptical because they believe their own positions are true to the claim that skeptics will not make reliable friends. As late as the 1980s, a number of scholars of ancient skepticism continued to maintain these claims, but opinion turned in the 1990s as a consensus emerged that skeptics could indeed live their skepticism, and that they would not necessarily be any more immoral than followers of other philosophies.

Much of Sextus's text consists of refutation of other dogmatic philosophies of the time. Since he quoted their ideas in order to refute them, his text has been an important source of information about ancient Stoicism, Epicureanism, and other philosophies.

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Science EncyclopediaScience & Philosophy: Semiotics to SmeltingSkepticism - Academic Skepticism, Pyrrhonism, Early Reception, Reception In And Since The Enlightenment, Skepticism In Medicine And Science