I argue that post-Kuhnian approaches to rational scientific change fail to appreciate several distinct philosophical requirements and relativist challenges that have been assumed to be, and may in fact be essential to any adequate conception of scientific rationality. These separate requirements and relativist challenges are clearly distinguished and motivated. My argument then focuses on Shapere's view that there are typically good reasons for scientific change. I argue: that contrary to his central aim, his account of good reasons ultimately presupposes the (...) requirement of universal standards of scientific reasoning; that the good reasons established by his account underdetermine the rationality of scientific change and allow that other changes would have been equally or even more rational; that as a result, Shapere's approach fails to meet what I characterize as the challenges of moderate, sociological, and cognitive relativism. (shrink)