"We speak of “Plato’s Theory,” and let me now say something about that. Its chief sources are, to be sure, the works of Plato, and he is its ultimate master. Yet within his works, the Dialogues, it is not Plato but his teacher Socrates who originates and maintains the theory. Plato presents Socrates as having a life-long hold on it, though he speaks of it under continuously changing aspects. There is a so-called “late” dialogue, the Parmenides, in which the elderly author imagines a boyish Socrates—a wonderful turnabout—and in which Socrates’ claim to authorship of the ideas is elicited by the father of philosophy, Parmenides, himself. (130 b) There is another dialogue, also written late in Plato’s life, the Sophist, in which an old Socrates, just a few weeks away from death, listens silently while a stranger brings the theory to its height with the solution of its deepest difficulty. And finally there is a “middle” dialogue, the Phaedo, in which Socrates, in the last conversation of his life, addresses the theory more directly than anywhere else. Plato, at least, wished the world to think of “Socrates’ Theory of Ideas.”
But then, more accurately, he would not have had us think of a “theory” at all. By a theory we usually mean a conceptual construction designed in principle to yield satisfying explanations for every problem brought before it. A theory ought to be falsifiable, which means it should be capable of being made to reveal its incompleteness or inconsistency by strenuous formal reasoning, so that, if need be, it may be discredited and discarded. Therefore it is its author’s responsibility to present it in the most impregnable form possible. Scholars do find such difficulties aplenty in the Theory of Ideas. But here is a curious circumstance: They are all anticipated in their boldest form in that very dialogue, the Parmenides, which represents a boyish Socrates as first proposing the Ideas. Can you think of another philosophical theory which is presented from the very outset in terms of a series of devastating difficulties, never to be explicitly resolved?"