The central focus of ancient Greek philosophy was the problem of motion . Many pre-Socratic philosophers thought that no logically coherent account of motion and change could be given. Although the problem was primarily a concern of metaphysics , not epistemology, it had the consequence that all major Greek philosophers held that knowledge must not itself change or be changeable in any respect. That requirement motivated Parmenides (flourished 5th century bce), for example, to hold that thinking is identical with “being” (i.e., all objects of thought exist and are unchanging) and that it is impossible to think of “nonbeing” or “becoming” in any way.

Plato accepted the Parmenidean constraint that knowledge must be unchanging. One consequence of that view, as Plato pointed out in the Theaetetus, is that sense experience cannot be a source of knowledge, because the objects apprehended through it are subject to change. To the extent that humans have knowledge, they attain it by transcending sense experience in order to discover unchanging objects through the exercise of reason.

Plato Plato, marble portrait bust, from an original of the 4th century bce ; in the Capitoline Museums, Rome. G. Dagli Orti—DeA Picture Library/Learning Pictures

The Platonic theory of knowledge thus contains two parts: first, an investigation into the nature of unchanging objects and, second, a discussion of how those objects can be known through reason. Of the many literary devices Plato used to illustrate his theory, the best known is the allegory of the cave, which appears in Book VII of the Republic. The allegory depicts people living in a cave, which represents the world of sense-experience. In the cave, people see only unreal objects, shadows, or images. Through a painful intellectual process, which involves the rejection and overcoming of the familiar sensible world, they begin an ascent out of the cave into reality. That process is the analogue of the exercise of reason, which allows one to apprehend unchanging objects and thus to acquire knowledge. The upward journey, which few people are able to complete, culminates in the direct vision of the Sun, which represents the source of knowledge.

Plato’s investigation of unchanging objects begins with the observation that every faculty of the mind apprehends a unique set of objects: hearing apprehends sounds, sight apprehends visual images, smell apprehends odours, and so on. Knowing also is a mental faculty, according to Plato, and therefore there must be a unique set of objects that it apprehends. Roughly speaking, those objects are the entities denoted by terms that can be used as predicates—e.g., “good,” “white,” and “triangle.” To say “This is a triangle,” for example, is to attribute a certain property, that of being a triangle, to a certain spatiotemporal object, such as a figure drawn in the sand. Plato is here distinguishing between specific triangles that are drawn, sketched, or painted and the common property they share, that of being triangular. Objects of the former kind, which he calls “particulars,” are always located somewhere in space and time—i.e., in the world of appearance. The property they share is a “form” or “idea” (though the latter term is not used in any psychological sense). Unlike particulars, forms do not exist in space and time; moreover, they do not change. They are thus the objects that one apprehends when one has knowledge.

Reason is used to discover unchanging forms through the method of dialectic, which Plato inherited from his teacher Socrates. The method involves a process of question and answer designed to elicit a “real definition.” By a real definition Plato means a set of necessary and sufficient conditions that exactly determine the entities to which a given concept applies. The entities to which the concept “being a brother” applies, for example, are determined by the concepts “being male” and “being a sibling”: it is both necessary and sufficient for a person to be a brother that he be male and a sibling. Anyone who grasps these conditions understands precisely what being a brother is.

In the Republic, Plato applies the dialectical method to the concept of justice. In response to a proposal by Cephalus that “justice” means the same as “honesty in word and deed,” Socrates points out that, under some conditions, it is just not to tell the truth or to repay debts. Suppose one borrows a weapon from a person who later loses his sanity. If the person then demands his weapon back in order to kill someone who is innocent, it would be just to lie to him, stating that one no longer had the weapon. Therefore, “justice” cannot mean the same as “honesty in word and deed.” By the technique of proposing one definition after another and subjecting each to possible counterexamples, Socrates attempts to discover a definition that cannot be refuted. In doing so he apprehends the form of justice, the common feature that all just things share.

Plato’s search for definitions and, thereby, forms is a search for knowledge. But how should knowledge in general be defined? In the Theaetetus Plato argues that, at a minimum, knowledge involves true belief. No one can know what is false. People may believe that they know something that is in fact false. But in that case they do not really know; they only think they know. Knowledge is more than simply true belief. Suppose that someone has a dream in April that there will be an earthquake in September and, on the basis of that dream, forms the belief that there will be an earthquake in September. Suppose also that in fact there is an earthquake in September. The person has a true belief about the earthquake but not knowledge of it. What the person lacks is a good reason to support that true belief. In a word, the person lacks justification. Using such arguments, Plato contends that knowledge is justified true belief.

Although there has been much disagreement about the nature of justification, the Platonic definition of knowledge was widely accepted until the mid-20th century, when the American philosopher Edmund L. Gettier produced a startling counterexample. Suppose that Kathy knows Oscar very well. Kathy is walking across the mall, and Oscar is walking behind her, out of sight. In front of her, Kathy sees someone walking toward her who looks exactly like Oscar. Unbeknownst to her, however, it is Oscar’s twin brother. Kathy forms the belief that Oscar is walking across the mall. Her belief is true, because Oscar is in fact walking across the mall (though she does not see him doing it). And her true belief seems to be justified, because the evidence she has for it is the same as the evidence she would have had if the person she had seen were really Oscar and not Oscar’s twin. In other words, if her belief that Oscar is walking across the mall is justified when the person she sees is Oscar, then it also must be justified when the person she sees is Oscar’s twin, because in both cases the evidence—the sight of an Oscar-like figure walking across the mall—is the same. Nonetheless, Kathy does not know that Oscar is walking across the mall. According to Gettier, the problem is that Kathy’s belief is not causally connected to its object (Oscar) in the right way.