Criticism invariably consists in pointing out some contradiction; either a contradiction within the theory criticized, or a contradiction between the theory and another theory which we have reason to accept, or a contradiction between theory and certain facts.… . Criticism is, in a very important sense, the main motive force of any intellectual development. Wthout contradictions, without criticism, there would be no rational motive for changing theories: there would be no intellectual progress. (Popper, 1963, p. 316)