 The first proposal is what he calls the Main Thesis: “The parts of a class are all and only its subclasses” (1991a: 7). By ‘class’ here, Lewis does not mean ‘set’. Classes are things with members. Some classes are proper classes, and hence not sets. And one set, the null set, has no members, so is not a class. Individuals, for Lewis, are things without members. Since the null set has no members, it is an individual. But the overlap between the sets and the classes is large; most sets we think about are classes.