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Many ancient cultures had sophisticated methods for organizing knowledge. However, systematic, self-conscious reflection on the nature of knowledge itself appears to have originated in Greek philosophy.
Greek philosophy began, around 600 b.c.e., with a series of attempts to specify the fundamental constituents of the universe. Questions soon began to be raised about the prospects for success in this enterprise. The poet-philosopher Xenophanes (c. 560–c. 478 b.c.e.) denied that knowledge—as opposed to belief or opinion—was possible, at least about the gods and the nature of the universe. However, since Xenophanes himself had much to say about gods, he must have considered some opinions more reliable than others; and other evidence seems to support this. Similarly, Democritus (c. 460–c. 370b.c.e.) expressed very serious doubts about our ability to know the real nature of things. But he too was the proponent of an ambitious cosmological theory—the original atomic theory—so he must have found room for some types of reasonable beliefs. He does speak of reason as superior to the senses, and able to go further than the senses alone. Yet he recognizes the need to rely on the senses as a starting point for inquiry, and this sets limits to how far anyone can repudiate the evidence they provide. How exactly he resolved this tension is debatable.
And as for what is clear, no man has seen it, nor will there be anyone
Who knows about the gods and what I say about all things;
For even if one should happen to say what has absolutely come to pass
Nonetheless one does not oneself know; but opinion has been constructed in all cases.
source: Xenophanes, fragment DK B34 (translation author's).
A sharp distinction between the senses and reason had earlier been drawn by Parmenides (born c. 515 b.c.e.), again to the detriment of the senses. But Parmenides also criticized those who relied on their senses as out of touch with true being, to which only pure reason can give access; the division between cognitive faculties is thus paralleled by a division between levels of reality—the ordinary world around us being stigmatized as less than fully real. This division is a prototype for Plato's distinction between purely intelligible Forms and the world perceived by the senses.
Socrates and Plato
Socrates (c. 470–399 b.c.e.), as portrayed in numerous dialogues of Plato (c. 428–348/347 b.c.e.), is famous for professing his ignorance. He does not say, as often claimed, that he knows he knows nothing, but that he knows he knows nothing truly valuable. How, then, does Socrates conceive the truly valuable knowledge that he lacks? A plausible answer is that it is knowledge of the nature of the human virtues—knowledge that, if one possessed it comprehensively, would amount to a quite general grasp of how to live our lives. In some of the same dialogues, Socrates also proposes a principle that has been called the Priority of Definition: unless one can provide a definition of a thing—unless one can specify what it is—one is in no position to say anything authoritative about that thing. It is difficult to see how inquiry can proceed if this principle is fully adopted, a problem that Plato has Socrates face in the Meno. Socrates' answer is the Pythagorean-influenced doctrine that "learning is recollection." We all have lived many past lives, and have knowledge buried within us; the trick is to reactivate this knowledge or bring it to the surface. The same dialogue also includes an account of the difference between knowledge and opinion; knowledge involves an ability to explain why things are as they are. Knowledge (or epistêmê, one of the words regularly translated "knowledge") thus seems to be a kind of systematic understanding of some subject-matter, as opposed to the mere awareness of isolated facts designated by the term "opinion."
The idea that learning is recollection recurs in other works of Plato, but in conjunction with the notion of separate, purely intelligible Forms. How exactly Plato conceives of Forms, and the motivations he has for postulating them, are controversial. But it is clear that each Form is thought of as encapsulating the being, or the essence, of the quality of which it is the Form; the Form of Beauty, for example, is the true nature of beauty, which particular beautiful objects in the world around us exemplify only in a limited or partial way. It is emphasized in the Republic that genuine knowledge is restricted to those who have a grasp of the Forms; anyone whose experience is limited to the everyday sensory world is only capable of opinion. To grasp Forms requires lengthy training, focused on minimizing one's reliance on the senses; a central tool in this process is pure mathematics.
The one dialogue of Plato devoted specifically to the question "What is knowledge?" is the Theaetetus ; but this, though probably a mature work, surprisingly contains no mention of Forms whatever. The three definitions of knowledge considered are "Knowledge is perception," "Knowledge is true judgment," and "Knowledge is true judgment plus an account." While all three definitions are rejected, the third seems to come closest to success; and this interestingly resembles the picture of knowledge as appropriately justified true belief, favored by many contemporary epistemologists.
Aristotle (384–322 b.c.e.) has comparatively little to say directly about knowledge. But it is clear that he too conceives of knowledge as involving systematic understanding. The findings of a properly developed science, he thinks, can ideally be laid out in connected sequences of explanations. The starting points are the natures or essences of the objects being studied—say, the essence of a cow; these natures or essences explain why the objects have certain features, which in turn explain why they have certain other features, and so on. Aristotle's remarks on how we come to know the starting points are somewhat baffling. What is clear is that sense perception is a crucial ingredient in the process of coming to know, but that sense perception by itself does not constitute knowledge. This is because sense perception shows us only particular objects; genuine knowledge is by definition about universal characteristics of things. One thus needs to be able to grasp the universal characteristics present in a body of related sensory information. Aristotle shows no lack of confidence in the ability of human beings to do this reliably. But this is no surprise; it is clear that he conceives of the world as ordered in such a way as to be understandable, and of human beings as having the capacities necessary to achieve that understanding—most notably, rationality. However, he stresses, particularly in his ethical works, that one cannot expect complete precision in all subjects; the study of ethics, no matter how expertly conducted, is bound to yield conclusions less exact and more subject to exceptions than the study of mathematics.
We think we know a thing without qualification … whenever we think we know the explanation because of which the thing is so, know that it is the explanation of that thing, and know that it does not admit of being otherwise.
source: Aristotle, Posterior Analytics, 71b9–12. In Aristotle: Selections, trans. Terence Irwin and Gail Fine.
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.
See also Empiricism ; Rationalism ; Skepticism .
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.