"Justice is the first virtue of social institutions, as truth is of systems of thought."
With these words John Rawls introduces, in his 1971 Theory of Justice, what he calls the "primacy of justice". Justice is understood here as a distributive concept defining principles for the "basic structure of society": any social arrangement that we would agree to as "free and equal persons" would pass the test of justice, which therefore, so conceived, sets out the terms of "social cooperation" in ways that do not violate that fundamental idea of equal freedom. And for justice to be the "first virtue" (primacy of justice) means that it cannot be compromised; specifically, justice cannot be sacrificed for the sake of greater wealth: "even the welfare of society as a whole cannot override" justice.
The comparison with systems of thought reflects a scientific paradigm calling for universal principles arrived at by agreement under strict logic and method. But no agreement can take place without first finding some common ground from which to reason, that is, without a "public conception of justice" or a "conception of social cooperation", and hence a "conception of society".