"For positivism, which has assumed the judicial office of enlightened reason, to speculate about intelligible worlds is no longer merely forbidden but senseless prattle. ... For the scientific temper, any deviation of thought from the business of manipulating the actual, any stepping outside the jurisdiction of existence, is no less senseless and self-destructive than it would be for the magician to step outside the magic circle drawn for his incantation; and in both cases violation of the taboo carries a heavy price for the offender."