Sometimes progress occurs as a consequence of logical clarification, as for instance, when a fallacy is exposed. (No one continues in the mistaken belief that the distorted faces in El Greco’s portraits were due to an astigmatic condition, once it has been explained that astigmatism would not lead to the painting of elongated faces. An astigmatic painter would perceive the faces on his canvas (and in the everyday world) to be elongated; but, in fact, these faces would appear completely normal to non-astigmatic eyes.) Sometimes progress results from dramatic scientific findings (the discoveries of Copernicus and Kepler radically changed our view about the architecture of the universe). And sometimes progress comes about when a large body of information is woven together in a convincing tapestry of argument (as happened when, in the course of introducing his theory of evolution, Charles Darwin reviewed masses of evidence about the development and differentiation of species).