One of the great virtues of knowledge of past ideas is that it forces present thinkers to work harder—not always an agreeable prospect. Hence the push to define the past as a realm of Stygian moral and intellectual darkness that our present knowing moral splendour has utterly superseded. Thus is current fashionable opinion both elevated and protected. Yet much that has been paraded in recent decades as cutting-edge thought is little more than ideas from as long ago as the fifth century BC re-packaged. Indeterminacy of meaning, for example—which the postmodernists make so much of—was a hot topic for Socrates…