As I wrote in For the New Intellectual: "To negate man's mind, it is the conceptual level of his consciousness that has to be invalidated. Under all the tortuous complexities, contradictions, equivocations, rationalizations of the post-Renaissance philosophy—the one consistent line, the fundamental that explains the rest, is: a concerted attack on man's conceptual faculty. Most philosophers did not intend to invalidate conceptual knowledge, but its defenders did more to destroy it than did its enemies. They were unable to offer a solution to the 'problem of universals,' that is: to define the nature and source of abstractions, to determine the relationship of concepts to perceptual data—and to prove the validity of scientific induction .... The philosophers were unable to refute the Witch Doctor's claim that their concepts were as arbitrary as his whims and that their scientific knowledge had no greater metaphysical validity than his revelations."
--Ayn Rand, Introduction to Objectivist Epistemology, Foreword to the First Edition