 According to Kuhn, these three interrelated aspects of incommensurability (changes in problems and standards that define a discipline, changes in the concepts used to state and solve those problems, and world change) jointly constrain the interpretation of scientific advance as cumulative. Scientific progress, Kuhn argued, is not simply the continual discovery of new facts duly explained. Instead, revolutions change what counts as the facts in the first place. When reigning theories are replaced by incommensurable challengers, the purported facts are re-described according to new and incompatible theoretical principles. The main goal of Kuhn’s Structure was to challenge the idea of scientific progress as cumulative, according to which what is corrected or discarded in the course of scientific advance is that which was never really scientific in the first place, and Kuhn used incommensurability as the basis of his challenge. Instead of understanding scientific progress as a process of change toward some fixed truth, Kuhn compared his suggestion to that of Darwin’s: scientific progress is like evolution in that its development should be understood without reference to a fixed, permanent goal (1962, 173).