Full Idea
Leibniz sets up increasingly stringent conditions possible worlds must meet. The weakest is non-contradiction, for truths of reason; then sufficient reason, for rational worlds; then identity of indiscernibles, for duplicates; then compossibility.
Gist of Idea
Leibniz narrows down God's options to one, by non-contradiction, sufficient reason, indiscernibles, compossibility
Source
report of Gottfried Leibniz (works [1690]) by Rom Harré - Laws of Nature 4
Book Reference
Harré,Rom: 'Laws of Nature' [Duckworth 1993], p.83
A Reaction
[my summary of a very nice two pages by Harré] God is the source of the principles which do the narrowing down.