 In “Anselm and Actuality” (1970a), Lewis tries to give as good a formulation of the  ontological argument  as can be made in modal realist terms. This is a good framework for discussing the ontological argument, since on one interpretation, the argument rests crucially on cross-world comparisons of greatness and the modal realist can make sense of that kind of talk better than views that reject possible objects. Lewis argues that the principle “A being than which nothing greater can be conceived is possible” is crucially ambiguous. One kind of reading is that the imagined being’s greatness in its world is greater than the greatness of any other being in that being’s world. That may be true, but it doesn’t imply that the being actually exists. Another kind of reading focusses on the imagined being’s greatness in this world. It says that there (actually) is a being whose actual greatness is greater than the greatness of any possible being. That entails the conclusion, but is not plausibly true. The broader conclusion here, that the ontological argument derives its persuasive force from an equivocation, is one that has been widely adopted since Lewis’s paper.