 Peter Forrest and D. M. Armstrong (1984) argue that modal realism leads to problems given the principle of recombination. An unrestricted principle of recombination says that for any things that could exist, there is a world in which there is a duplicate of all of them. Forrest and Armstrong apply the principle by taking the things to be the different possible worlds. A world containing a duplicate of all the worlds would, they show, be bigger than any world. But by the principle it would also be a world. Contradiction. Lewis’ reply is to deny the unrestricted version of the principle. He insists that there is independent reason to qualify the principle to those things whose size and shape permits them to be fit into a single world. Without an unrestricted principle of recombination, there is no way to create the large world that’s at the heart of Forrest and Armstrong’s paradox.