Abstract

Conceivability is, I say, prima facie evidence for possibility. Hence, we may count the cost of theories about possibility by listing the ways in which, according to the theory in question, something conceivable is said nonetheless to be impossible. More succinctly we may state a principle, Hume's razor to put alongside Ockham's. Hume's razor says that necessities are not to be multiplied more than necessary. In this paper I count the cost of David Lewis's modal realism, showing that many of the objections are replied to by Lewis only at the cost of multiplying necessities.