In 1934 when Gaston Bachelard published his Nouvel Esprit Scientifique  and Karl Popper’s Logik der Forschung  appeared few philosophers would have dissented from the view that science develops in a linear or monistic fashion, so as to leave meaning and truth-value unchanged, on the basis provided by common experience. Meyerson had even undertaken to show that the theory of relativity could be deduced from Newtonian principles  and it was widely held that, for their part, the concepts of classical physics were just a refinement of the concepts of daily life.  Since then Bachelard, in France, and Popper, in England, have been more than any others responsible for the seeping into the general philosophical consciousness (which includes the consciousness of scientists in their reflection upon their work) of the fact, profoundly revolutionary for philosophy, of the phenomenon of scientific discontinuity (with respect to common sense or experience) and change. In strikingly similar terms Bachelard and Popper attempted to register this phenomenon. Yet neither of them, nor the theoretical traditions they inaugurated, have succeeded in grasping its full significance for philosophy. Dominique Lecourt’s Marxism and Epistemology  and Paul Feyerabend’s Against Method  constitute in a sense extended commentaries on these traditions and their attempts to theorize scientific discontinuity and change—the one, a respectful tribute ‘from outside’; the other, a ‘wicked’ polemic from within.