 And during the 1960s and 1970s, Popper indicated that, according to his “critical approach” to science and philosophy, all science should be revolutionary—revolution in permanence. But this was a tame conception of revolution compared to Kuhn’s, given Popper’s two logical criteria for a progressive new theory: (a) it must logically conflict with its predecessor and overthrow it; yet (b) “a new theory, however revolutionary, must always be able to explain fully the success of its predecessor” (Popper 1975). As we shall see, Kuhn’s model of revolution rejects both these constraints (depending on how one interprets his incommensurability claim) as well as the idea of progress toward final, big, theoretical truths about the universe. Kuhn dismissed Popper’s notion of revolution in perpetuity as a contradiction in terms, on the ground that a revolution is something that overthrows a long and well–established order, in violation of the rules of that order. Kuhn (1970) also vehemently rejected Popper’s doctrine of falsification, which implied that a theory could be rejected in isolation, without anything to replace it. According to Popper, at any time there may be several competing theories being proposed and subsequently refuted by failed empirical tests—rather like several balloons being launched, over time, and then being shot down, one by one. Popper’s view thus faces the difficulty, among others, of explaining the long-term coherence that historians find in scientific research.