Neither Plato nor Aristotle was particularly concerned with how knowledge is possible, or with warding off doubts on that score. In the Hellenistic period (roughly, the last three centuries B.C.E.) this topic became much more prominent; this was also the period of the organized skeptical movements. The Stoics shared Aristotle's conception of epistêmê as involving a systematic understanding of a body of truths. But they were also much concerned with the notion of an "apprehensive appearance," which is an impression, sensory or otherwise, that somehow guarantees its own correctness. It is not entirely clear how this was supposed to work. Some evidence suggests that the Stoics considered such impressions to have an inherent clarity or distinctness that left no room for error; but the guarantee of truth may instead have been regarded as due to their having been appropriately caused by their objects. The leading members of the Academy (the school founded by Plato) in this period relentlessly attacked the idea of "apprehensive appearances"; they also argued that nothing of the sort was necessary for living a reasonable human life.
The Epicureans, in the same period, also seem to have been concerned with minimizing error. They strikingly claimed that "all perceptions are true," and that error occurs only in our interpretations of them. However, this seems to be bound up with the atomist theory of sense perception, in which objects give off constant streams of atoms that enter our eyes (or other sense organs). "All perceptions are true" in the sense that there is no possibility of error concerning the configuration of atoms that strikes the sense organ. But that configuration need not accurately represent the shape of the original object; the film of atoms given off by a square tower, for example, may be eroded in transit, so that it is round when it reaches one's eye. There is therefore no guarantee that we perceive the world as it really is. Nonetheless, this theory does have the resources to explain how we manage to be mostly correct about the world around us, while also explaining why we sometimes make mistakes.
The Academics were not the only skeptical movement in Greek philosophy. There was also the Pyrrhonist movement (claiming inspiration from Pyrrho (c. 365–c. 275 B.C.E.), which began in the first century B.C.E. but is best known through the writings of Sextus Empiricus (probably 2nd century C.E.). According to Sextus, the skeptic suspends judgment on all questions about the nature of things, because of the "equal strength" of the opposing arguments and impressions available on any given topic. Sextus also claims that this posture results in ataraxia, "freedom from disturbance"; the stakes, for a skeptic, are simply much lower than for everyone else.
Aristotle. Aristotle: Selections. Translated, with introduction, notes, and glossary, by Terence Irwin and Gail Fine. Indianapolis: Hackett, 1995.
——. Posterior Analytics. 2nd ed. Translated with a commentary by Jonathan Barnes. Oxford: Clarendon Press, 1993.
Inwood, Brad, and L. P. Gerson, eds. Hellenistic Philosophy: Introductory Readings. 2nd ed. Indianapolis: Hackett, 1997.
McKirahan, Richard D., Jr. Philosophy before Socrates: An Introduction with Texts and Commentary. Indianapolis: Hackett, 1994.
Plato. Complete Works. Edited, with introduction and notes, by John M. Cooper. Indianapolis: Hackett, 1997.
Burnyeat, Myles, and Michael Frede. The Original Sceptics: A Controversy. Indianapolis: Hackett, 1997.
Everson, Stephen, ed. Epistemology. Cambridge, U.K., and New York: Cambridge University Press, 1990. White, Nicholas. Plato on Knowledge and Reality. Indianapolis: Hackett, 1976.
Science EncyclopediaScience & Philosophy: Ephemeris to Evolution - Historical BackgroundEpistemology - Ancient - Pre-socratic Philosophy, Socrates And Plato, Aristotle, Hellenistic Theories, Bibliography