 Nozick’s sensitivity condition is a subjunctive conditional: if p were false, S would not believe it. The usual way in which such conditionals are evaluated is by assuming that there is an ordering of possible worlds according to how much they resemble the actual world. A subjunctive conditional \(A \rightarrow B\) is true if and only if B is true in the closest (or all the closest) possible worlds where A is true. According to this semantics, subjunctive conditionals do not contrapose (the contrapositive of a conditional if A, B is if not-B, not-A). Thus, suppose that we flip a coin to decide whether you or I will strike this match: heads you strike it, tails I do. The coin comes up head, you strike the match and it lights. In this situation, it is true that if I had struck the match, it would have lit. But it doesn’t seem to be true that if the match hadn’t lit then I wouldn’t have struck it. The match might have failed to lit because it was wet while either of us struck it. In the possible worlds terminology, the closest possible world where I strike the match is a world where it lights, but there are possible worlds where the match doesn’t light and I strike it that are as close to actuality as are worlds where the match doesn’t light and you strike it.