Deflationism about the necessary a posteriori and Twin Earth

Some necessary truths are a posteriori. That’s widely agreed and is presumed here. Their existence might appear to show that discoveries about how things are in fact—about how things actually are—can lead to discoveries about all the ways things might be, about the nature of logical space. I detail one way of resisting this conclusion for a number of examples, and the implications of Twin Earth for the issue. Central is the notion of a Cambridge discovery.