 This entry has been stressing Lewis’s many and diverse contributions to philosophy. But there is one thesis with which he is associated above all others: modal realism. Lewis held that this world was just one among many like it. A proposition, p is possibly true if and only if p is true in one of these worlds. Relatedly, he held that individuals like you or I (or this computer) only exist in one possible world. So what it is for a proposition like You are happy to be true in another world is not for you to be happy in that world; you aren’t in that world. Rather, it is for your counterpart to be happy in that world.