Chapter 5 of Tim Williamson's new book The Philosophy of Philosophy argues that modal knowledge is a species of counterfactual knowledge. Why should we believe this? The only reason offered is that there is a logical equivalence between modal claims and certain counterfactuals. In Williamson's words (p. 25):
"Given that the equivalences … are logically true, metaphysically modal thinking is logically equivalent to a special case of counterfactual thinking, and the epistemology of the former is tantamount to a special case of the epistemology of the latter."
But it does not follow from the bare fact that modal claims are logically equivalent to certain counterfactual claims that modal epistemology is tantamount to a special case of counterfactual epistemology. By analogy, it does not follow from the fact that that disjunctive propositions AvB are equivalent to negated conjunctive propositions ~(~A&~B) that the epistemology of disjunctive propositions is a special case of the epistemology of negated propositions. Nor does it follow from the fact that atomic propositions A are logically equivalent to conjunctive propositions A&A that the epistemology of atomic propositions is a special case of the epistemology of conjunctive propositions.
The equivalences cited by Williamson cannot by themselves establish that knowing certain counterfactuals is the way – or even our usual way – of knowing modal facts. At most, they might be taken to suggest that knowing the relevant counterfactuals is a way of knowing modal facts. I take it that epistemologists of 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.
Moreover, without supplementation, flagging the mere existence of logical equivalences is not even to specify a way of coming to know about modality. We are left wondering what the supposed way of knowing is supposed to be like. Is it envisaged that we know the modal claims by first knowing the counterfactual claims and then deriving the modal claims which are equivalent to them? If so, it looks very unlikely that many of us are using this route to modal knowledge much of the time. Most people could not work through the relevant derivations if they tried, and even those who could certainly don't seem to be doing that kind of thing very often. On the other hand, if the envisaged route to modal knowledge does not go via derivation from the equivalent counterfactuals, what is it like? And what is the epistemological significance of the equivalence?