Plato (c. 427–347 B.C.) was the pupil of Socrates (470–399 B.C.), and Aristotle (384–322 B.C.) was the pupil of Plato.
Socrates was not interested in arguing with his fellow Athenians merely for the sake of argument—as the Sophists were—but rather he wanted to discover the essential nature of knowledge, justice, beauty, goodness, and the virtues (such as courage).
The Socratic (dialectic) method: a search for the proper definition of a thing that will not permit refutation under Socratic questioning. The method does not imply that the questioner already knows the proper definition, only that the questioner is skilled at detecting misconceptions and at revealing them by asking the right questions.
Socrates was famous for his courage and for his staunch opposition to injustice. The story of his trial and subsequent death by drinking hemlock after his conviction for “corrupting” young men and not believing in the city’s gods is told in Plato’s dialogues Apology, Crito, and Phaedo.
Plato’s metaphysics: the Theory of Forms. What is truly real is not the objects of sensory experience but the Forms or Ideas. These are not just in the head but are in a separate realm and are ageless, eternal, unchanging, unmoving, and indivisible. Circularity and beauty are examples of Forms.
Particular objects have a lesser reality that can only approximate the ultimate reality of Forms.
A thing is beautiful only to the extent it participates in the Form beauty, and is circular only if it participates in the Form circularity.
Plato introduced into Western thought a two-realms concept of a “sensible,” changing world (a
Student’s Guide | Chap. 3: Socrates, Plato
source of error, illusion, and ignorance) and a world of Forms that is unchanging (the source of all reality and all true knowledge). This Platonic dualism was incorporated into Christianity and today still affects our views on virtually every subject.
Some Forms, notably the Forms truth, beauty, and goodness, are of a higher order than other
Forms. The Form circularity is beautiful, but the Form beauty is not circular.
Plato’s theory of knowledge. Plato developed the first comprehensive theory of knowledge in philosophy, though many of his predecessors had implicit epistemological theories, some of them based in skepticism.
10. A skeptic is someone who doubts that knowledge is possible. Xenophanes declared that even if truth were stated it would not be known. Heraclitus believed that just as one cannot step into the same river twice, everything is in flux; though he himself did not deduce skeptical conclusions from his metaphysical theory, it does suggest that it is impossible to discover any fixed truth beyond what the theory itself expresses. Cratylus argued that a person cannot step even once into the same river because the person and the river are continually changing. True communication is impossible since words change their meaning even as they are spoken. It seems to follow that knowledge would also be impossible.
11. The Sophists, who could make a plausible case for any position, seemed to support skepticism by implicitly teaching that one idea is as valid as the next. Gorgias said there was no reality, and if there were, it could not be known or communicated; Protagoras, the best- known Sophist philosopher, maintained that “man is the measure of all things.” Plato took this to mean that there is no absolute knowledge because one person’s view of the world is as valid as any other’s.
12. In his dialogue Theaetetus, Plato argued that if Protagoras were correct, the person who viewed
Protagoras’s theory as false would also be correct. Plato also argued against the popular notion that knowledge can be equated with sense perception. Knowledge clearly involves thinking and the use of concepts that cut across individual sense perceptions. Knowledge can be retained even after a person’s