 Then Thomas Kuhn and Paul Feyerabend challenged received views of science and made talk of revolutionary breaks and incommensurability central to the emerging new field of history and philosophy of science. They asserted that major conceptual changes lay in the future of mature, modern sciences as well as in their past. Kuhn (1962, ch. IX) contended that there will be no end to scientific revolutions as long as systematic scientific investigation continues, for they are a necessary vehicle of ongoing scientific progress–necessary to break out of dated conceptual frameworks. In other words, there are both founding revolutions, in something like Butterfield’s sense of threshold events to maturity, and a never-ending series of later revolutions within an ongoing field, no matter how mature it is. However, soon after Structure, Kuhn had second thoughts and eventually abandoned the Butterfield conception of revolution, on the ground that even his so-called preparadigm schools had their paradigms (Kuhn 1974, 460, note 4; details below). So multiple Kuhnian paradigms in long-term competition now became possible.