Full Idea
An ersatz world must be maximally consistent (hence destroyed by an additional sentences), …but that is prima facie a modal distinction: a set of sentences is consistent iff those sentences, as interpreted, could all be true together.
Clarification
'prima facie' means at first glance
Gist of Idea
Maximal consistency for a world seems a modal distinction, concerning what could be true together
Source
David Lewis (On the Plurality of Worlds [1986], 3.2)
Book Reference
Lewis,David: 'On the Plurality of Worlds' [Blackwell 2001], p.151
A Reaction
This is indicative of Lewis's motivation for his project, which is to eliminate modal facts from the world. Only a vast multitude of non-modal concrete worlds can satisfy all the contraints. Cf many-worlds quantum mechanics for non-locality.