Full Idea
Lewis's different systems of modal logic differed about such formulae as □P implies □□P; ◊□P implies □P; and ◊S implies □◊S
Clarification
'Nec' (usually a square) and 'poss' (a diamond) are necessarily and possibly
Gist of Idea
The main modal logics disagree over three key formulae
Source
Stephen Yablo (Apriority and Existence [2000], §06)
Book Reference
'New Essays on the A Priori', ed/tr. Boghossian,P /Peacocke,C [OUP 2000], p.203
A Reaction
Yablo's point is that the various version don't seem to make much difference to our practices in logic, mathematics and science. The problem, says Yablo, is deciding exactly what you mean by 'necessarily' and 'possibly'.