Full Idea
In the possible worlds account modus ponens is validated (the closest world, the actual, is a B-world just if B is true), and modus tollens is validated (if B is false, the actual world is not an A-world, so A is false).
Gist of Idea
In the possible worlds account of conditionals, modus ponens and modus tollens are validated
Source
Frank Jackson (Conditionals [2006], 'Famous')
Book Reference
'Blackwell Guide to Philosophy of Language', ed/tr. Devitt,M/Hanley,R [Blackwell 2006], p.216
A Reaction
[see Jackson for slightly fuller versions] This looks like a minimal requirement for a decent theory of conditionals, so Jackson explains the attractions of the possible worlds view very persuasively.