In a recent essay in this Journal, 'The Value of a Fixed Methodology,'' John Worrall has taken me to task for having claimed in my Science and Values [1984], that not only have the theories of science changed through time but so alike have the methods and aims of science. He and I disagree both about the factual claim that the methods have shifted and about our appraisals of the philosophical significance of such shifts. This dispute goes well beyond differences between Worrall and me about historical and philosophical matters. It is one of the two or three central issues which divide philosophers in the so-called historical school, both from other camps of philosophy and amongst themselves. This debate pits Popper against Kuhn, Lakatos against Toulmin, McMullin against Shapere, and Worrall against me. Because the topic is thus of relatively broad interest and provenance, I want to respond briefly to some of Worrall's criticisms of my work on this score. (1) Worrall is disturbed by the prospect that the methods, aims and standards of the scientific enterprise might change through time. I shall, in due course, present some reasons to think not only that they do change, but that something would be very bizarre about the scientific enterprise if they did not. However, before I deal with that question, there is a prior issue which must be grappled with. Worrall makes it vividly clear that what really frightens him about the prospect of changes in scientific rationality is that such changes, in his view, open the floodgates to relativism. As he puts it at one point: 'If no principles of evaluation stay fixed, then there is no 'objective viewpoint' from which we can show that progress occurred ... However this is dressed up, it is relativism'. (p. 274) Again, he says that 'without such an ('invariant core of methodological principles') the model (viz., Laudan's) collapses into relativism'. (p. 275) And early on in his essay, he insisted that 'laying down fixed principles of scientific theory-appraisal is the only alternative to relativism'. (p. 265). The initial point I wish to make is that Worrall has wholly misconstrued the threat from relativism. The central claim of the epistemic relativist, at least where standards and methods are concerned, is not that those standards change but that-whether changing or unchanging-those standards have no independent, non-question begging rationale or foundation. Even if man had been using exactly the same inferential principles ever