In his book The Idea of Justice, Amartya Sen argues that we do not need to know what perfect justice is to know that a particular state of affairs is unjust, and comparatively more unjust than some other state of affairs. (This is the Nontranscendental Thesis, pp. 5–7, 15–17.) Analogously, he says, we do not need to know what the perfect picture would be in order to know whether a particular Picasso is worse than a particular Van Gogh (pp. 15, 101). He also argues that we can do incomplete rankings, knowing the answers to some questions about comparative justice without being able to answer them all, and we can engage in judgments relating multiple factors without being paralyzed by incommensurability. In terms of action, he argues for eliminating particular recognizable injustices rather than pursuing perfect justice. His discussion, therefore, is about nonideal rather than ideal justice, both in claiming to do without an ideal theory and in not pursuing an ideal in action.
Kamm, Frances. "Sen on Justice and Rights: A Review Essay." Review of The Idea of Justice, by Amartya Sen. Philosophy and Public Affairs, 39.1, March 2011: 82-104.