Full Idea
The new discovery was that with a suitable structure imposed on the space of possible worlds, the Leibnizian idea would work for all modal logics.
Gist of Idea
It was realised that possible worlds covered all modal logics, if they had a structure
Source
Michael Dummett (Could There Be Unicorns? [1983], 1)
Book Reference
Dummett,Michael: 'The Seas of Language' [OUP 1993], p.329