Full Idea
Things have modal properties only relative to the choice of a counterpart relation, and the choice between alternative counterpart relations is not constrained by the metaphysics.
Gist of Idea
Modal properties depend on the choice of a counterpart, which is unconstrained by metaphysics
Source
Robert C. Stalnaker (Mere Possibilities [2012], 3.6)
Book Reference
Stalnaker,Robert C.: 'Mere Possibilities' [Princeton 2012], p.71
A Reaction
Stalnaker is sympathetic to counterparts, but this strikes me as a powerful objection to the theory. I take the modal properties of something to be fixed by its actuality.