Should our factual understanding of the world influence our normative theorising about it? G.A. Cohen has argued that our ultimate normative principles should not be constrained by facts. Many others have defended or are committed to various versions or subsets of that claim. In this paper I dispute those positions by arguing that, in order to resist the conclusion that ultimate normative principles rest on facts about possibility or conceivability, one has to embrace an unsatisfactory account of how principles generate (...) normative political judgments. So political theorists have to choose between principles ostensibly unbiased by our current understanding of human motivation and political reality, or principles capable of reliably generating political judgments. I conclude with wider methodological observations in defence of the latter option, and so of a return to political philosophy’s traditional blend of normative and descriptive elements. (shrink)