G. A. Cohen, answered from the texts and cited to the page.
Fundamental principles of justice are fact- insensitive: when one's belief in a principle P rests on a fact F, one must be able to specify the more fundamental fact-insensitive principle P* that, together with F, generates P — and the regress terminates at fact- insensitive principles. This is the Clarity of Mind Requirement. Facts enter the construction of justice not at the level of fundamental principles but at the level of rules of regulation — the principles that tell us, given facts about human nature, scarcity, technology, and political feasibility, how to implement fundamental justice in institutional design.1
The Rawlsian constructivism of A Theory of Justice conflates principles of justice with rules of regulation: it derives the two principles of justice from the original-position deliberation under the veil of ignorance, where the deliberators reason from facts about the circumstances of justice and the conditions of human cooperation.2 That derivation is the derivation of rules of regulation for a society with the relevant facts; it is not the derivation of justice itself.
Justice itself is what would be true regardless of whether the circumstances of justice obtain; the circumstances of justice limit what can be required of justice, not what justice is.3
Now we do, of course, need such information when selecting prin- Rescuing Justice from... 10. If I am right that constructivists miscast fundamental principles of justice in the role of principles of social regulation, what, I may be asked, is the (contrasting and) proper fundamental principles justice? answer proprietary role, apart obvious spelling justice is.Rescuing Justice and Equality, pp. 284–285
And, similarly, Humean justice, the observance of rules of promise-keeping and property and so (though not much) on, is what it is independently of whether circumstances make it necessary and/or possible, and independently of whether or not they make judgments of justice appropriate, and Hume gives no sign of thinking otherwise. In a situation which promises precarious property unsustainable, Rescuing Justice from.Rescuing Justice and Equality, pp. 334–335
These people believe that all sound principles are, as I shall say, fact-sensitive, by which, once again, I mean neither more nor less than that facts form least grounds affirming them.Rescuing Justice and Equality, pp. 231–232
The fact-insensitivity thesis and the constructivism critique are among the most controversial of my late positions; the Boston Review symposium on Rescuing Justice and Equality and the journal responses by Joshua Cohen, Andrew Williams, David Estlund, Liam Murphy, and Thomas Pogge pressed the thesis hard. The principal objections are: that the regress argument against fact-sensitivity is unsound because the specification of P is in many cases not possible (Estlund); that the principles/rules-of-regulation distinction is artificial (Williams, Murphy); that Rawlsian constructivism is consistent with fact-insensitivity once properly understood (Joshua Cohen); and that the position conflates metaethics (what justice is) with political philosophy (what we ought politically to do). My reply, defended in the General Appendix to Rescuing Justice and Equality*, is that the regress argument survives the objections and that the principles/rules-of-regulation distinction is not artificial but tracks a real and significant difference in the role facts play at each level.