Throughout MIE, we are told what incompatibility is. Two claims are incompatible when commitment to one precludes entitlement to the other. We are also told that incompatibility is a modal notion. In fact, it plays are rather central and rather modal role in the later part of MIE and Brandom’s later work. Now, my problem is I don’t understand where the modality comes from. Commitment and entitlement are normative but they aren’t, at least at this point in the story, modal. The preclusion isn’t modal either; it is just straightforward non-modal precluding.

There is a modal sense of incompatibility that is used in other philosophical papers. This is, I think, a sense in which two propositions are not jointly possible; non-compossible is the term I think. This is clearly modal. If that is what Brandom means, then there should be some demonstration that this sort of incompatibility and the kind defined in MIE coincide. There isn’t any such demonstration, which makes me think that this is not on the right track. However, if it isn’t on the right track, then I don’t know how in the world incompatibility is modal. If it is on the right track though, then I’m also not sure how the argument is supposed to run since these two notions don’t seem coextensional. I’m doubtful that the latter implies the former either, but then this is just denying that I’m on the right track.