Evidence for presuppositions

One of the things evidential challenges do is to question what grounds we have for our antecedent beliefs. Indeed, that's one of the main things they do. Maybe it's common knowledge that we all believe that p, but we haven't thought much about why we all believe that p. This is just the sort of case where it might make sense to stop and wonder whether p—which is mutually recognized to be something we believe—is actually supported by our evidence.

I think that this observation undermines a claim Michael Blome-Tillmann makes in his book Knowledge & Presuppositions. The key idea there, as in his earlier paper of the same title, is to embrace a Lewisian from of contextualism about 'knows', but where Lewis's 'Rule of Actuality' is replaced by a 'Rule of Presupposition'—that which is consistent with the presuppositions in a conversation is not properly ignored. Ad Michael characterizes conversational presuppositions in terms of dispositions reflective of mutual belief.

Michael considers (p. 99) this dialogue:

A: I know that animal is a zebra.
B: How do you know that it isn't a mule cleverly painted to look like a zebra?
A: Hmm, for all I know it is a painted mule. So I was wrong. I didn't know that it is a zebra after all.

In a strategy unlike the canonical one (supposing that final A's self-attribution of error is mistaken), Michael thinks that A's final response is wholly correct, and that the initial utterance is false. He thinks that the painted mule hypothesis is relevant, even at the start of the conversation. Here's why:

[B] does not pragmatically presuppose that the animals are not cleverly painted mules—neither before nor after asking her first question. Since B asks whether A can rule out that the animal is a cleverly painted mule, B is clearly not disposed to behave, in her use of language, as if she believed it to be common ground that the animal is not a cleverly painted mule. For if she were so disposed, she would certainly not have asked that very question. (100)

This seems to me simply to be wrong, for the reason laid out in my opening paragraph. Asking whether p is ruled out by evidence seems to me wholly consistent with thinking it to be common ground that p.

(Although I think it's independent, this point points to the same conclusion, and has an argumentative similarity with, the one I press against Michael's view in this paper.)