Counterfactuals is David Lewis' forceful presentation of and sustained argument for a particular view about propositions which express contrary to fact conditionals, including his famous defense of realism about possible worlds and his theory of laws of nature.

It has been maintained that we are quite able to express (1*)–(4*) without the introduction of a dyadic deontic operator, provided only that we supply our standard deontic logic with a stronger conditional than material implication. The lesson learned from Chisholm's paradox has been the eminently convincing, indeed obvious, one: that what we ought to do is not determined by what is the case in some perfect world, but by what is the case in the best world we can ‘get (...) to’ from this world. What we ought to do depends upon how we are circumstanced. (shrink)