This article criticizes Laura Valentini's criterion for distinguishing good and bad idealizations in normative political theory. I argue that, on an attentive reading of her criterion, all ideal theories she discusses must be written off as incorporating bad idealizations. This fact makes Valentini's criterion trivially implausible, for it is argued that there are good idealizations that succeed in promoting the action-guiding goal of ideal theory. Upon rejecting an attempt to salvage the idealizations that Valentini marks off as bad, I develop an alternative criterion for demarcating good and bad idealizations. The criterion holds that the standing of a theory's idealized assumptions depends on whether the stipulated idealizations can be feasibly realized in the non-ideal world, and thus on whether the principles that the theory generates can be made relevant for real-world practice. I also claim that the feasibility criterion better reflects the function of idealization in promoting action-guidance. Unlike Valentini's criterion, the feasibility criterion yields the result that Rawls' theories of domestic and international justice both incorporate bad idealizations.