Nihil Obstat: Lewis's compatibility account of abilities

In an outline of a paper found amongst his philosophical papers and correspondence after his untimely death in 2001—‘Nihil Obstat: An Analysis of Ability’, reproduced in this volume—David Lewis sketched a new compatibilist account of abilities, according to which someone is able to A if and only if there is no obstacle to their A-ing, where an obstacle is a ‘robust preventer’ of their A-ing. In this paper, we provide some background context for Lewis’s outline, a section-by-section commentary, and a general discussion of the account’s main features.