Contra John Rawls, G. A. Cohen argues that the fundamental principles of justice are not constrained by the limits of our nature or the nature of society, even at its historical best. Justice is what it is, even if it will never be realized, fully or at all. Likewise, David Estlund argues that since our innate motivations can be justice-tainting, they cannot be a constraint on the right conception of justice. Cohen and Estlund agree that if the attempt to implement a certain conception of justice is likely to result in widespread harm or injustice, then it should not be implemented, but that this does not entail that the conception itself is false. I argue that (i) there is no way to judge the soundness of a principle of justice independently of all psychological facts, and the effects that the principle is likely to have if it is implemented; (ii) a principle of justice that, if implemented, makes it hard or impossible for individuals committed to justice to lead happy and worthwhile lives, even if the circumstances are favorable to living justly, cannot be sound; (iii) without the constraints noted in (i) and (ii), there can be no reason to reject racist, sexist, or other wrongheaded principles of justice that have been advanced as sound over the years, principles that even Cohen and Estlund would reject. In short, justice is justice only if kept within the limits of human nature.