Abstract: It is argued that some possible worlds are only contingently possible, and likewise that some impossible worlds are only contingently impossible. There are counterexamples to S5 modal logic and even to S4. What about B? Although there are no convincing grounds for supposing that even the actual world is only contingently possible, as far as the logic of modality is concerned (as opposed to the metaphysical reality), the actual world might instead have been impossible. The correct modal propositional logic is the simple system T.

Keywords: actuality, Lewis, logic, modality, necessity, possibility, world,