Full Idea
Leibniz argued that each monad mirrors or expresses every monad with which it is compossible. Hence compossibility is an equivalence relation among monads; possible worlds may then be identified as the corresponding equivalence classes.
Gist of Idea
Each monad expresses all its compatible monads; a possible world is the resulting equivalence class
Source
report of Gottfried Leibniz (works [1690]) by Ian Rumfitt - The Boundary Stones of Thought 6.1
Book Reference
Rumfitt,Ian: 'The Boundary Stones of Thought' [OUP 2015], p.154
A Reaction
[Rumfitt cites Benson Mates 1986:IV.1 for this claim] There is an analogous world of all the human minds that are in communication with one another - something like a 'culture'.