 In general, if explanation is directional, it may not seem surprising that when A explains B, it is not usually the case that B, or its negation, is in turn an explanation of A (or its negation). Audrey’s winning the race explains our celebration, but our failure to celebrate is not (normally) a plausible explanation of her failure to win. Solange’s presence may explain why the seminar was such a great success, but a boring seminar is not—in any normal set of circumstances—a reason why Solange is not at it. This result seems to undermine the usual understanding that if A is a sufficient condition of B, it will typically be the case that B is a necessary condition for A, and the falsity of B a sufficient condition for the falsity of A.