Full Idea
The ways in which a given thing can be different in different possible worlds depend on the ways in which such a thing can be different at different times in the actual world.
Gist of Idea
Possible difference across worlds depends on difference across time in the actual world
Source
Sydney Shoemaker (Causality and Properties [1980], §05)
Book Reference
Shoemaker,Sydney: 'Identity, Cause and Mind' [OUP 2003], p.218
A Reaction
Where change in a thing is possible across time in the actual world seems to require a combination of experiment and imagination. Unimaginability does not entail necessity, but it may be the best guide we have got.