Lewis's dilemma of explanation under indeterminism exposed and resolved

In a brief passage, David Lewis derives from quantum-theory a dilemma regarding the explanation of chance events which he tries to solve by first distinguishing plain from contrastive why-questions, and then arguing that with respect to chance events, only plain why-questions have answers. His brevity warrants elaboration and critique. I endorse his derivation, but I make a structural objection to his solution. Once a further distinction is drawn between different kinds of contrastive why-question, his solution can be modified and refined so as to go some way to meeting this objection. However, it cannot resist a deeper objection to the effect that chance events can be given contrastive explanations after all. The alternative solution to the dilemma that I recommend holds Lewis' conception of the "essence of chance" to be illusory.