 Philosophers as diverse as Immanuel Kant and John Stuart Mill have argued that unless two options are deliberatively commensurable, in this sense, it is impossible to choose rationally between them. Interestingly, Kant limited this claim to the domain of prudential considerations, recognizing moral reasoning as invoking considerations incommensurable with those of prudence. For Mill, this claim formed an important part of his argument that there must be some one, ultimate “umpire” principle – namely, on his view, the principle of utility. Henry Sidgwick elaborated Mill’s argument and helpfully made explicit its crucial assumption, which he called the “principle of superior validity” (Sidgwick 1981; cf. Schneewind 1977). This is the principle that conflict between distinct moral or practical considerations can be rationally resolved only on the basis of some third principle or consideration that is both more general and more firmly warranted than the two initial competitors. From this assumption, one can readily build an argument for the rational necessity not merely of local deliberative commensurability, but of a global deliberative commensurability that, like Mill and Sidgwick, accepts just one ultimate umpire principle (cf. Richardson 1994, chap. 6). 