There are important interpretive questions that are too easily overlooked. The question I'm after right now is just: what in particular is Williamson trying to accomplish in this material? I think that many people are not always clear about this question. (Williamson is one of these people.)
One candidate project is to answer what I'll call the 'how' question:
How do we have modal knowledge?
A possible answer to the how question suggested by Williamson's work, which emphasizes the equivalence of modal claims with certain counterfactuals, is that we acquire modal knowledge by coming to have counterfactual knowledge, and exploiting the connection between counterfactual truths and modal truths. I think understanding Williamson's project in this way would be a mistake.
It is easy to interpret Williamson as addressing the how question. Carrie Jenkins does so in her critique of Williamson, considering whether Williamson succeeds in showing that "knowing certain counterfactuals is the way, or even our usual way, of knowing modal facts." She quite rightly points out that establishing the logical equivalence of modal claims with certain counterfactuals does not show anything about how humans in fact come to modal knowledge. Logical equivalence isn't psychological equivalence.
This is, to my mind, a pretty compelling objection to Williamson qua answerer of the how question. But it's such an obvious objection that it makes me nervous about attributing that aim to Williamson. Notice, for instance, that Williamson himself emphasizes that he's not making a claim about psychological equivalence, and suggests that this is not relevant to his concerns. He writes:
Indeed, we have no sufficient reason to regard any of the equivalences [i.e., necessarily A iff, were not-A, A] as strict synonymies. That detracts little from their philosophical significance, for failure of strict synonymy does not imply failure of logical equivalence. The main philosophical concerns about possibility and necessity apply equally to anything logically equivalent to possibility or necessity. (160)
Williamson is not as clear as one might like about what makes something a 'main philosophical concern'. Jenkins thinks the claim here is just false -- that "epistemologists about modality are (rightly) more centrally interested in the question of how we do know about modality than in the question of how we might know about modality," and therefore that Williamson dramatically misses the mark. I'm not sure she's right about what epistemologists are rightly centrally interested in -- I have my doubts. (Ben Jarvis and I develop some related ideas in our book.) That's not my present issue. I'd like to argue today that this question -- the how question -- isn't the one that centrally interests Williamson, and isn't the one he's attempting to answer. Williamson says that the epistemology of metaphysics is a subset of the epistemology of counterfactuals. This obscure phrase needn't -- and, I think, shouldn't -- be interpreted as the claim that one typically comes to modal knowledge by inferring modal facts from counterfactual knowledge.
Williamson motivates the issue, in the introductory sections of his chapter, with a puzzle. (The following remarks are, in my view, pretty close to a paraphrase of §1 of the chapter.) He begins with an expression of the standard view that (a) we have, in many cases, knowledge of metaphysical modality, and (b) the relevant truths of metaphysical modality are mind-independent. Our ability for knowledge in this domain, therefore, is an interesting and noteworthy accomplishment that calls for explanation. But why should we have developed this ability? Particularly if, as some of the extant literature on modal epistemology seems to assume, our capacity to reason with metaphysical modality is to a significant degree sui generis, it begins to look rather mysterious. After all, our ancestors faced no evolutionary pressure towards having true beliefs about metaphysical possibility and impossibility. So positing a special capacity for modal knowledge, even if correct -- even if it correctly answered the how question -- would not dissolve the present mystery. The present mystery is not a how question but a why question:
Why are we able to form reliable beliefs about metaphysical modality?
Williamson's answer is that the capacity for knowledge of metaphysical modality -- that which stands in need of explanation -- is provided by the capacity for knowledge of counterfactuals -- something which is less mysterious, as (he argues) there is a sensible evolutionary story to be told about how it came to be.
This is a legitimate form of an answer to the why question, although of course it does not answer the how question. But I don't see that Williamson is trying to answer the how question.
I'm not at present taking a stand on whether Williamson's answer is one we should accept.