Published online by Cambridge University Press: 01 January 2022
I commemorate David Lewis by discussing an aspect of modality within analytical mechanics, which is closely related to his work on counterfactuals. This concerns the way Hamilton-Jacobi theory uses ensembles, i.e. sets of possible initial conditions. (A companion paper discusses other aspects of modality in analytical mechanics that are equally related to Lewis's work.)
Many thanks to: Peter Holland and Graeme Segal for conversations; Alexander Afriat, Robert Bishop, Larry Gould, Susan Sterrett and Paul Teller for comments on a previous version; and Gerard Emch and Klaas Landsman for technical help.