Full Idea
If T is only possible if S obtains, and S is possible but doesn't obtain, then T is only possible in the world where S obtains, but T is not possible in the actual world. It follows that the relation of relative possibility is not transitive.
Gist of Idea
If something is only possible relative to another possibility, the possibility relation is not transitive
Source
Michael Dummett (Could There Be Unicorns? [1983], 1)
Book Reference
Dummett,Michael: 'The Seas of Language' [OUP 1993], p.330
A Reaction
[compressed]
Related Idea
Idea 16953 Relative possibility one way may be impossible coming back, so it isn't symmetrical [Dummett]