Eve has a follow-up to the post I linked yesterday. This is the part that gets closest to the sort of argument I had in mind:

Basically many of [my interlocutors] say, "You can use sectarian arguments, but only when there could maybe be secular reasons in there somewhere." But who decides which reasons are sufficiently secular–and why?

If I can dredge up one atheist who thinks we need to keep "under God" in the Pledge of Allegiance because it reminds us to be humble, even though (let’s say; I’m making this up) 99% of the people who want the phrase in the Pledge are God-fearin’ believers… is this an acceptable public-square sectarian argument? How many atheists do I have to convince of any particular position (from "gay marriage is an oxymoron" to–what should be far more controversial–"human nature has no history") before it’s sufficiently secular? How many atheists before it’s Febreezed?

And which sects and anti-sects are sufficiently far apart? If a Cat’lick and a Prot agree on something, is that no longer sectarian? What about a Catholic, a Jew, and a Muslim? How close to "…walk into a bar" territory do we have to get before an argument is considered broadly-enough-accepted?

(Does it matter if the Catholic is Camille Paglia, the Jew is Naomi Wolf, and the Muslim is Irshad Manji? Because I bet I could get that lot to agree to some whacked-out things.)

If it’s, "I know it when I see it," well that’s fair enough, but it’s hard for me when I see something different. If you think "nature" or whatever is a Jesus word that contaminates your pristine politics, I really need you to argue for that (and try to convince me on my ground from your premises, the same way I’m doing from my side) and not just assert that I’m out of bounds for saying the taboo word.

I need you to tell me what makes your abstractions boringly obvious and mine scarily sectarian, and so far, no argument I’ve seen has convinced me that this can be settled a) without reference to metaphysical beliefs or b) faster than we’d settle things if you just let me argue politics in whatever way comes naturally.

This is an especially knotty problem for my opponents because my whole claim is that our culture conditions us to find some claims obvious and other claims risible, and those divisions don’t match up well with the truth.

To come at it from a slightly different angle, suppose you could, as Eve and I both think you can’t, give a non-arbitrary specification of which notions are and aren’t sufficiently detached from a “comprehensive” picture of the world to count as secular in the relevant sense. Any argument, then, that passes muster in the public square must be one that employs notions taken only from this class. But you still have to say more than this: for example, is it required for an argument to pass muster that it be a logically valid argument? (Probably.) A sound one? (No, because soundness requires the truth of the premise and it’s exactly our incapacity to agree on this latter thing that’s gotten us into this predicament.) Must it appear to be sound? This seems reasonable enough – but appear that way to whom? And under what circumstances? If I and my co-religionists get together in private and use our shared comprehensive doctrines to hammer out a political position that we all agree on, and then we emerge into the public square and offer a “secularized” argument for that position that’s only convincing to us, have we done enough? If we make up a majority of the population, we can then vote our position into law; does the mere attempt at appealing to “public” reason in support of our view entitle us to do this? Why not?

Note that my claim is emphatically not that politics should be unmoored from the art of persuasion: and in any society as ideologically diverse as our own, any argument that borrows on a lot of narrowly sectarian vocabulary (or on bald appeals to “special revelation”, as a commenter put it yesterday) isn’t likely to do a good job of creating a democratic consensus and will therefore need to be supplemented by other sorts of arguments for that reason. But then Eve’s point (and mine) is just that in such a society the goodness of a public argument is best measured by its persuasiveness, and that the kind of sterilization that the secularist demands is very often not the best way to achieve this: to return to a point that Eve makes earlier in that follow-up post and that I alluded to yesterday, our moral concepts, and so also our standards for determining whether moral arguments are convincing or not, are far too bound up with our dread comprehensive views of the world to make such an idealized picture a workable one.

Elsewhere: James on a similar dilemma for Damon Linker.