Full Idea
To tell the alleged truth about contradictory things that happen on a mountain is just contradicting yourself, but you can't tell the truth by contradicting yourself. There is no mountain where contradictions are true, and impossible worlds are no better.
Gist of Idea
On mountains or in worlds, reporting contradictions is contradictory, so no such truths can be reported
Source
David Lewis (On the Plurality of Worlds [1986], 1.2 n3)
Book Reference
Lewis,David: 'On the Plurality of Worlds' [Blackwell 2001], p.7
A Reaction
[compressed] He says this works for any 'restricted' domain like a mountain or a real world, but that it wouldn't apply in an unrestricted fictional world.
Related Idea
Idea 16133 Possible worlds can contain contradictions if such worlds are seen as fictions [Lewis]