Political philosophy offers a range of utopian proposals, from open borders to global egalitarianism. Some object that these proposals ought to be constrained by what is feasible, while others insist that what justice demands does not depend on what we can bring about. Currently, this debate is mired in disputes over the fundamental nature of justice and the ultimate purpose of political philosophy. I take a different approach, proposing that we should consider which facts could fill out a feasibility requirement. (...) This search for the facts requires requires looking to the social sciences, but I argue that it turns out that the social sciences will not provide us with findings that rule out, nor even count against, the kinds of proposals that political philosophers actually make, whether ideal or non-ideal. At the least, to deny this requires adopting deeply controversial commitments within the philosophy of social science. Thus, I conclude that a feasibility requirement has little practical use for political philosophers. Disputes over that requirement ought to be replaced by other, more fruitful ways for political philosophers to address both the findings of social science and the debates over non-ideal theory or political realism. (shrink)