I believe that almost all important, useful ideas are simple. Peter Whittle has recently put it nicely in an autobiographical essay. "If a piece of work is heavy and complicated then it is wrong." . . . Some writers feel that to express their ideas in simple terms is degrading. Some use complexity to disguise the paucity of their material. In fact, simplicity is a virtue and when, as here, it is both original and useful, it can represent a real advance in knowledge.