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date: 23 March 2019

knowledge, theories of

Summary and Keywords

Questions about the nature and possibility of knowledge extend throughout Greek philosophy. In the early period, several thinkers raised doubts about our ability to know the truth of the proto-scientific theories they themselves were developing. Plato depicted Socrates as disclaiming knowledge about anything important but searching for fundamental ethical truths. He (Plato) also introduced the idea of unchanging Forms, a grasp of which is crucial for knowledge; in one dialogue, he examined a number of proposed definitions of knowledge itself. Aristotle developed an ideal of scientific knowledge centered on demonstrations of why the objects under examination must have certain features, the starting points of which are an understanding of the essences of the things in question. The Stoics and the Epicureans both offered robustly positive accounts of how knowledge is possible, and they were challenged on this by sceptics of both the Academic and Pyrrhonian traditions.

Keywords: knowledge, opinion, epistemology, philosophy, scepticism, senses, reason, science, demonstration

A number of ancient cultures had highly developed methods for organizing knowledge. However, it was in ancient Greek philosophy that systematic, self-conscious reflection on the nature of knowledge itself appears to have begun. It is not clear that we can speak of fully worked out theories of knowledge prior to Aristotle, although this impression may be partly due to our limited evidence. In any case, the question of what we can know, and how, is almost as old as Greek philosophy itself—at least, on the traditional view that puts Thales of Miletus (active around 585 bce) as the first Greek philosopher.

Early Greek Philosophy

Thales was the first of a succession of Greek thinkers to attempt to identify the basic constituents of the world and the processes that work upon them. It was not long before such proto-cosmology faced the question “how do you know?” A four-line fragment of the poet-philosopher Xenophanes (c. 570–c. 478 bce) denies that anyone can have knowledge, as opposed to opinion or belief (dokos, LM 8D49). This has often been taken as expressing a thoroughgoing epistemological scepticism, but there are two reasons for doubting this. First, knowledge is said to be impossible “about the gods and what I say about all things”—that is, about theology and cosmology (“all things”), not knowledge-claims in general. Second, since the surviving fragments contain numerous speculations on both these subjects, Xenophanes cannot have thought the impossibility of knowledge fundamentally undermined attempts to discover the truth; another fragment, which looks like a comment on his own writings, reads “Let these things be believed as like the truth” (LM 8D50), apparently suggesting that some opinions can be better than others.

A similar combination of doubts about knowledge and positive theorizing is apparent in the fragments of Democritus (c. 460–c. 370 bce), the other early cosmologist of whom we have significant epistemological reflections. Democritus was one of the originators of the atomic theory, but this itself generates worries about our knowledge of the world. Since the world does not present itself to us in ordinary experience as consisting of atoms and void, it follows that we do not see things as they really are. And yet we cannot but rely on our senses in developing our views of how things really are. One fragment (LM 27D23a) depicts the senses addressing the mind, warning it that overthrowing the evidence they provide will destroy its own credibility; another (LM 27D20–21) asserts that the mind can go further than the senses in accuracy or precision. There is a delicate balance here, but the two thoughts can perhaps be combined if one conceives the atomic theory as developed by rational reflection starting from, but transcending, sensory evidence, yet as also able to explain why things appear to the senses as they do, so that sensory evidence is not wholly repudiated. Even so, several fragments point out our inability to know “how each thing is” (LM 27D16, 19, cf. 18); while the atomic theory in general can be posited as a correct account, we are not in a position to discern the precise atomic configuration of each object.

In sharply distinguishing the senses and reason, Democritus was following the lead of Parmenides (born c. 515 bce). But Parmenides’ disparagement of the senses goes much further; we are to ignore the deliverances of the senses and to rely on reason instead. This is because only reason, and not the senses, can give us access to pure being; the world shown to us by the senses is dominated by things coming into being and passing out of being, and changing in all kinds of ways—in other words, by processes that involve non-being. The relative ranking of the senses and reason is thus paralleled by a relative ranking of levels of reality. Despite this, even Parmenides offered a cosmology—complete with a warning that it was deceptive (LM 19D8.52). The detailed interpretation of Parmenides’ thought is highly debatable. But it seems clear that his division of cognitive faculties, and his accompanying postulation of a level of reality superior to that discernible by the senses, were an important influence on Plato.

Socrates and Plato

Socrates (469–399 bce) wrote nothing, and for our impressions of him we are dependent mainly on the works of Plato (424/3–347 bce) and Xenophon. A conspicuous aspect of Plato’s portrayal of him in several dialogues, most notably the Apology, is his profession of ignorance. It has been claimed since antiquity that Socrates said in Plato’s Apology that he knew he knew nothing. What he actually says is that he does not know anything important or valuable (kalon k’agathon, literally “fine and good,” 21d3–4). This, of course, raises the question of what the truly valuable knowledge that he (like everyone else) lacks might be. His account of what he urges everyone to concern themselves with in the Apology, coupled with his eager, but invariably unsuccessful, attempts in numerous other dialogues to answer questions of the form “What is F?”—where F is some virtue or other evaluative quality—suggest that it is knowledge of the real nature of virtues such as piety, courage, or moderation. In the Euthyphro, Socrates says that if he could grasp the nature of piety, he could “use it as a model” to determine whether any given action was pious or not (6e4-5). It sounds as if, were this procedure to be generalized, it would yield a comprehensive means of deciding how to live our lives. If one adds to this the thesis that “virtue is knowledge,” to which Socrates seems to be attracted in several dialogues, and which seems to eliminate any gap between knowing the nature of a virtue and exemplifying it, this knowledge would guarantee our actually living well. Valuable knowledge indeed, if only one could have it; and this explains why Socrates never seems to falter in his quest, despite repeated failures.

In some of these same dialogues (how many is a matter of some dispute), Socrates relies on a principle that has become known in the scholarship as the Priority of Definition: unless one can provide a definition of some object F—that is, answer the “what is F?” question—one does not have knowledge of anything else about F. This principle, if taken really seriously, seems to pose severe, even insuperable obstacles to any kind of inquiry. In the Meno, where the principle is as conspicuous as anywhere, Plato has Socrates face up to this problem. Socrates’ response is the extraordinary idea that “learning is recollection.” We have all lived many past lives (this seems to represent a Pythagorean influence on Plato), and have in fact acquired all knowledge. But it is buried within us and needs to be reactivated or brought to our awareness; hence, what we call learning is really just recollecting what we already knew. It is difficult to understand how to take this: if there really is a problem with inquiry in the present life, why was there not just the same problem in all the previous lives? In any case, one consequence of this account is that the sum total of things available to be known is static; the idea of genuinely new knowledge being created over time seems to be ruled out. The view itself can be regarded as a remote ancestor of the early modern conception of innate ideas, and its successors.

The Meno also includes an explicit account of how knowledge differs from opinion—specifically, correct opinion. The latter may be just as useful for practical purposes as the former. But knowledge is superior to opinion in that it includes the ability to explain why things are as they are, and this gives it a permanence and stability that opinion, being an awareness of isolated facts, cannot aspire to (97e6-98a8). A natural way to read this is to understand knowledge (or at least epistêmê, one of the terms usually translated as “knowledge,” and the one that occurs here) as a kind of systematic grasp of some subject matter, such as the general principles of geometry.

The conception of knowledge as static is taken further in Plato’s (Parmenides-influenced) notion of unchanging, purely intelligible Forms, which exist separately from the objects in the ordinary world that exhibit them. These Forms appear in a number of dialogues, but it is not clear that there is any one consistent or fully developed “theory of Forms.” What exactly there are Forms of (the “population” of the world of Forms), and why Plato thinks we need to believe in them, are very difficult questions. But this much would be generally agreed: Forms are the essential natures of whatever they are Forms of. Thus, the Form of Beauty (to take one of Plato’s favorite examples) is the essential nature of Beauty. Particular beautiful objects in the ordinary world exemplify this (or “participate” in it—the nature of this relation is also obscure) and thus warrant the label “beautiful”; but they do so only in a limited or defective way, owing to their inherent variability. In the Republic, knowledge and opinion are again contrasted, and here knowledge is limited to the philosophers, that is, those who have an understanding of the Forms; if one’s awareness is restricted to the sensory realm, one cannot rise above opinion. Coming to understand the Forms is an arduous process; a key element is getting away from dependence on the senses, which makes mathematics a crucial preliminary study. In the Republic—although this is not mentioned in any other dialogue—the Form of the Good is identified as the highest Form of all, with the others dependent on it for their being, and knowledge of them dependent on knowledge of the Good. What this amounts to is one of the most difficult questions in Platonic interpretation.

One Platonic dialogue, the Theaetetus, is wholly devoted to the question “what is knowledge?”. It is one of Plato’s deepest and most challenging works and was almost certainly written at a time after he had formulated the idea of separately existing Forms. It is striking, then, that the Theaetetus includes no explicit mention of Forms whatever, although opinions differ on the extent to which, in its various failed attempts to define knowledge, it is pointing to the need for Forms. Three definitions are considered: (a) knowledge is perception—incorporating an interpretation and critique of the Sophist Protagoras’ famous assertion, “A human being is measure of all things,” although whether Protagoras meant this as an epistemological claim is impossible to say; (b) knowledge is true opinion; and (c) knowledge is true opinion plus an account. The third, which is verbally close to the view offered in the Meno, fails because of the speakers’ inability to formulate a satisfactory picture of what an account might be; but again, arguably Plato is pushing us to improve on them. In any case, “true opinion plus an account” looks like a forerunner of the notion of knowledge as justified true belief, which has been the focus of much discussion in recent epistemology.


Aristotle (384–322 bce) does not spend much time talking directly about the nature of knowledge. But it is clear from a number of works, especially the Posterior Analytics, that he has a considered view on the matter. Aristotle takes from Plato the idea of epistêmê (in Aristotle sometimes translated as “science” rather than “knowledge”) as a systematic understanding of some subject matter. A fully mature science can be displayed in a precise structure consisting of chains of explanations—what he calls demonstration. The starting points of these explanations are the natures or essences of the sciences’ objects—in the case of biology, for example, the essences of the various animals; these will explain why the animals have certain features, and those in turn explain why they have other features, etc. Aristotle does not imagine that he has reached this point in any of the sciences to which he devotes his attention. But he is very clear that knowledge by definition involves grasping the reason why things are the way they are, and that they could not be otherwise (An. Post. 71b9–12); and this ultimately derives from the essential natures of the things being studied, which for Aristotle are fixed features of the world.

Aristotle’s account of how we come to grasp the starting-points—which, of course, are not themselves susceptible to further explanation—occurs in the very difficult final chapter of Posterior Analytics (2.19). The starting point is sense perception (an anti-Platonic move). But sense perception cannot itself be a form of knowledge, because it grasps only particulars, and knowledge (or science) has to do with universals—the species cow, for example, rather than individual cows. One therefore needs to acquire multiple memories of related sense perceptions; these then coalesce in what Aristotle calls experience (empeiria), and from here a grasp of the starting points is possible. This may make it sound far too easy. But Aristotle’s biological works paint a robust picture of empeiria; it includes, for example, looking inside bodies, which presupposes skilled dissection. The cluster of memories included in empeiria therefore goes far beyond those acquired, say, from seeing a number of cows in fields; once one has a decent level of empeiria, so understood, one is arguably not too far from discerning the cow’s essential nature.

There is, nonetheless, a striking epistemological optimism in Aristotle. The world is orderly and understandable, and we are the kinds of being who are capable (with enough hard work, of course) of understanding it. He is simply not troubled by the question whether knowledge is even possible. On the other hand, there are differences in the level of precision that one can expect in different subjects. Ethics, in particular, is singled out as a field in which one’s conclusions are necessarily not as exact or exceptionless as in, say, mathematics.

The Hellenistic Period

In this period, the question of the very possibility of knowledge is taken much more seriously. The Stoics and the Epicureans both offer accounts strongly defending its possibility, while members of the Academy, which in this period was dominated by a sceptical outlook, raised significant doubts about it. The Stoics extend Plato’s and Aristotle’s tendency to think of epistêmê as systematic; for them, it is the entire set of interconnected truths embodied in the wise person, the ideal figure toward whom the Stoics aspire. But epistêmê is formed out of individual “apprehensive appearances” (phantasiai katalêptikai). The apprehensive appearance is an impression—often, but not always, a sensory impression—that could not be mistaken; apprehensive appearances can thus be thought of as a kind of knowledge, in a more mundane contemporary sense of the term. While the wise person is distinguished by assenting only to “apprehensive appearances,” all of us routinely experience them. The force of this “could not be mistaken” is controversial. Some evidence suggests that the Stoics took these impressions to include some phenomenally accessible mark guaranteeing their correctness. But it is hard to see what this might be, and numerous scholars have argued that the guarantee of correctness lies instead in the impressions’ causal history: they are caused by their objects in such a way that there is no room for error to creep in. It is possible that the Stoics themselves did not sufficiently clarify the nature of the guarantee. In any case, the two leading sceptical Academics, Arcesilaus (316/5–242/1 bce) and Carneades (214/3–129/8 bce), argued at length against the possibility of “apprehensive appearances”—and against the idea that they were needed for living a satisfactory human life.

The Epicureans made the striking claim that “all perceptions are true,” and that error arises only in our interpretations of them. The Epicureans were atomists, and they conceived of sense perception as a process in which objects constantly emit atomic images of themselves, images that penetrate our sense organs. A given configuration of atoms striking the sense organ carries no room for error, and in that sense “all perceptions are true.” However, it does not follow that one will invariably perceive the external object as it actually is, because the shape of the atomic image entering the perceiver may not be the same as that of the original object; a favorite example is a square tower that looks round at a distance, because the corners of the atomic film emanating from the tower have been worn down by the time they reach one’s eye. This makes “all perceptions are true” rather less reassuring than it sounded, if one’s goal is to acquire knowledge of the external world. Still, the Epicurean view allows us to explain why we sometimes fail to perceive things as they are, even though it also makes sense of the idea that we mostly get things right.

The other sceptical tradition in Greek philosophy was Pyrrhonism, which claimed inspiration from Pyrrhon (c. 365–275 bce), but as a self-conscious movement began in the 1st century bce. Its only surviving writings are the works of Sextus Empiricus (2nd or early 3rd century ce). According to Sextus, the sceptic suspends judgement on all questions concerning how things really are, as a result of the “equal strength” (isostheneia) of the considerations for and against any given position. Scepticism is not specially about epistemology; in principle, any subject is grist for the sceptic’s mill. However, Sextus’ critical arguments frequently raise questions about the justification of positions under discussion. His Five Modes, summarized in book 1 of his Outlines of Pyrrhonism (PH 1.164–177), but employed widely in contexts of justification, offer a comprehensive challenge to any form of foundationalist epistemology; in recent philosophy, the three most important of these Modes have received considerable interest under the heading “The Agrippan Trilemma” (after Agrippa, their originator according to Diogenes Laertius, 9.88).

Sextus was a doctor, as were several other Pyrrhonists; and there are interesting connections between Pyrrhonist scepticism and the Empiricist approach to medicine, which avoided all speculation about the inner workings of the body and concentrated solely on routines that had been shown by experience to be effective in curing diseases or healing wounds. But epistemological debates within medicine were just as vigorous as in philosophy more generally, one of the most ardent anti-sceptics being Galen (129–c. 215 ce).

Late Antiquity

The philosophy of late antiquity was dominated by Aristotelianism and by Platonism; writings frequently took the form of commentaries on the works of the founder. Platonic and Aristotelian elements often occur in combination, and there is a measure of influence from Stoicism, even though the school itself did not survive beyond about 200 ce. The most important Platonists are Plotinus (205–269/70) and Proclus (412–485); on the Aristotelian side, the most important figure is perhaps Alexander of Aphrodisias (active around 200). Whether Aristotle or Plato would have recognized themselves in these writings is debatable; on the Platonic side, questions concerning knowledge are very hard to consider in isolation from a metaphysical outlook arrived at by highly creative reading of certain suggestions in Plato.

Primary Texts

Aristotle, Posterior Analytics (2nd ed.). Translated with commentary by Jonathan Barnes. Oxford: Oxford University Press, 1993.Find this resource:

Burnyeat, Myles, and M. J. Levett. The Theaetetus of Plato. Indianapolis, IN: Hackett, 1990.Find this resource:

Dillon, John, and Lloyd P. Gerson. Neoplatonic Philosophy: Introductory Readings. Indianapolis, IN: Hackett, 2004.Find this resource:

Laks, André and Glenn Most, ed. and trans., Early Greek Philosophy. 9 vols. Cambridge, MA: Harvard University Press, 2016 (LM).Find this resource:

Long, Anthony A., and David N. Sedley. The Hellenistic Philosophers. Vol. 1, Translations of the Principal Sources with Philosophical Commentary. Cambridge: Cambridge University Press, 1987.Find this resource:

Long, Anthony A., and David N. Sedley. The Hellenistic Philosophers. Vol. 2, Greek and Latin Texts with Notes and Bibliography. Cambridge: Cambridge University Press, 1987.Find this resource:

Plato, Five Dialogues: Euthyphro, Apology, Crito, Meno, Phaedo, translated by G.M.A. Grube, revised by John M. Cooper (Indianapolis/Cambridge: Hackett Publishing, 2nd ed., 2002).Find this resource:

Sextus Empiricus. Outlines of Scepticism. Edited and translated by Julia Annas and Jonathan Barnes. Cambridge: Cambridge University Press, 2000. (Originally titled Outlines of Pyrrhonism.)Find this resource:

Stanford Encyclopedia of Philosophy. The following articles (listed in the order in which their topics are mentioned in this article) are especially relevant: “Xenophanes,” “Democritus,” “Parmenides,” “Plato,” “Plato’s Middle Period Metaphysics and Epistemology,” “Plato on Knowledge in the Theaetetus,” “Aristotle” (esp. sections 3 & 4), “Aristotle’s Biology,” “Aristotle’s Logic” (esp. section 6), “Stoicism” (esp. section 4), “Arcesilaus,” “Carneades,” “Epicurus,” “Ancient Skepticism,” “Sextus Empiricus,” “Commentators on Aristotle,” and “Neoplatonism.”


Algra, Keimpe, Jonathan Barnes, Jaap Mansfeld, and Malcolm Schofield, eds. The Cambridge History of Hellenistic Philosophy, Part III, “Epistemology.” Cambridge, U.K.: Cambridge University Press, 1999.Find this resource:

Asmis, Elizabeth, “Epicurean Empiricism.” In The Cambridge Companion to Epicureanism. Edited by James Warren, 84–104. Cambridge, U.K.: Cambridge University Press, 2009.Find this resource:

Bett, Richard. “Socratic Ignorance.” In The Cambridge Companion to Socrates. Edited by Donald Morrison, 215–236. Cambridge, U.K.: Cambridge University Press, 2011.Find this resource:

Bolton, Robert. “Aristotle: Epistemology and Methodology.” In The Blackwell Guide to Ancient Philosophy. Edited by Christopher Shields, 151–162. Malden, MA: Blackwell, 2003.Find this resource:

Burnyeat, Myles, and Michael Frede, eds. The Original Sceptics: A Controversy. Indianapolis, IN: Hackett, 1997.Find this resource:

Everson, Stephen, ed. Epistemology: Companions to Ancient Thought 1. Cambridge, U.K.: Cambridge University Press, 1990.Find this resource:

Gerson, Lloyd. Ancient Epistemology. Cambridge, U.K.: Cambridge University Press, 2009.Find this resource:

Hankinson, R. “Stoic Epistemology.” In The Cambridge Companion to the Stoics. Edited by Brad Inwood, 59–84. Cambridge, U.K.: Cambridge University Press, 2003.Find this resource:

Lee, Mi-Kyoung. Epistemology after Protagoras: Responses to Relativism in Plato, Aristotle, and Democritus. Oxford: Oxford University Press, 2005.Find this resource:

Lesher, J. H. “Early Interest in Knowledge.” In The Cambridge Companion to Early Greek Philosophy. Edited by A. A. Long, 225–249. Cambridge, U.K.: Cambridge University Press, 1999.Find this resource:

Schwab, Whitney. “Explanation in the Epistemology of the Meno.” Oxford Studies in Ancient Philosophy 48 (2015): 1–36.Find this resource:

Sedley, David. The Midwife of Platonism: Text and Subtext in Plato’s Theaetetus. Oxford: Oxford University Press, 2004.Find this resource:

Sedley, David. “Philosophy, Forms, and the Art of Ruling.” In The Cambridge Companion to Plato’s Republic. Edited by G. R. F. Ferrari, 256–283. Cambridge, U.K.: Cambridge University Press, 2007.Find this resource:

Striker, Gisela. Essays on Hellenistic Epistemology and Ethics. Cambridge, U.K.: Cambridge University Press, 1996.Find this resource:

Vogt, Katja Maria. Belief and Truth: A Skeptic Reading of Plato. New York: Oxford University Press, 2012.Find this resource:

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