This post is an exercise in Williamson exegesis. I'm looking primarily at chapter five -- the modal epistemology chapter -- of The Philosophy of Philosophy. (That chapter substantially overlaps a couple of earlier papers as well.) As many readers will know, Williamson emphasises the equivalence of claims of metaphysical modality with particular counterfactuals (such as the ones discussed in my recent post here), and suggests, therefore, that our ordinary imagination-based capacity for the evaluation of counterfactual conditionals brings along with it a capacity for knowledge of metaphysical modality. As Williamson says, "the epistemology of metaphysical thinking is tantamount to a special case of the epistemology of counterfactual thinking."
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.