I've been meaning to comment on Matt's skeptical post about public reason. In a nutshell, Matt worries that folks are offering arguments in publicly acceptable terms in “bad faith.” That is, they are motivated by their comprehensive views, which they know not everyone shares, and so they give an argument that is more broadly acceptable, even if they don't really believe it.
. . . I am totally unconvinced that the “public reason” arguments against gay marriage are being offered in good faith. I know perfectly well that people who oppose gay marriage do so overwhelmingly either out of religious conviction or simple prejudice. I also know that people understand that such arguments cannot be presented in elite media contexts and so forth and that, as Beinart writes, it's considered (even by people who haven't studied Political Liberalism) to be a kind of 'debate foul' to just pound the Bible. So they gone out into the world, searching around for a tolerable public reason argument that will reach their favored conclusion. But the motivating issue for (the vast majority of) these people isn't demographic shifts in Scandinavia, it's the religion stupid. As a result, I have little incentive to take the empirical arguments offered by the anti-gay folks, and as a result of that, they have little reason to bother to make their arguments convincing (since they know no one will be convinced no matter what) rather than simply providing a kind of “public reason” cover for their real agenda.
I think Matt's largely right as a matter of fact, but the relevance of Matt's observation isn't clear. Rawls is doing a bit of what he calls “ideal theory,” and although I don't think Rawls conceives of ideal theory in the right way, it remains that norms of public reason are offered by Rawls as part of an ideal normative conception of a well-ordered society, and not as a description of actual norms of public discourse. The observation that people, as a matter of descriptive fact, offer public reasons in “bad faith” has the same standing as the observation that people, as a matter of descriptive fact, only grudgingly pay their taxes under the threat of sanction. That, of course, does not mean that it is okay for people to pay their taxes only grudgingly. It may be that they ought to recognize and be motivated by a duty of justice.
Rawls at one point talks about the way a society might move from a mere modus vivendi (a kind of truce or detente) to an order that is stable “for the right reasons,” i.e., because enough people have come to affirm a public conception of justice. Relatedly, we don't just begin with full-fledged norms of public reason. These must develop over time. The fact that many folks recognize that some “arguments cannot be presented in elite media contexts and so forth and that, as Beinart writes, it's considered (even by people who haven't studied Political Liberalism) to be a kind of 'debate foul' to just pound the Bible” is a very important step on the path toward more widespread and robust norms of public reason.
I think we should consider it important to reinforce these norms, to make sure that people know what is an is not a debate foul. This is not something worth doing just for its own sake. For one thing, we need to do it so that we can have a debate at all, and not just the assertion and counter-assertion of incommensurable considerations. But mainly, we need to do it to reinforce the ideal of a pluralistic liberal order. We have to remind people, and keep reminding them, and keep reminding them, that we do not all agree on certain fundamental matters, and that therefore we should agree to refrain from using politics to impose our vision on others.
Now, I agree with Matt that the ideal of public reason is “in a great deal of tension with human nature.” I therefore don't think that we can realize an order that is stable for the “right” Rawlsian reasons. The best we can hope for is some kind of modus vivendi. But this kind of stability need not be fragile. It can be fairly robust and self-equilibrating, and is quite worth having. It is unlikely that even most people will ever internalize norms of public reason. But if enough of the right kind of people do so, that can have a deeply positive effect on the neutrality and stability of our social order. So keep the faith.