 Now, as was the case with numbers, properties, and sentences, not everyone who endorses possible worlds thinks that they are abstract objects; indeed, one leading proponent of the use of possible worlds in philosophy and semantics — namely, David Lewis (1986) — maintains that possible worlds are of the same kind as the actual world, and so he takes them to be concrete objects. However, most philosophers who endorse possible worlds take them to be abstract objects (see, e.g., Plantinga (1974, 1976), Adams (1974), Chisholm (1976), and Pollock (1984)). It is important to note, however, that possible worlds are very often not taken to constitute a new kind of abstract object. For instance, it is very popular to maintain that a possible world is just a set of propositions. (To see how a set of propositions could serve as a possible world, notice that if you believed in full-blown possible worlds — worlds that are just like the actual world in kind — then you would say that corresponding to each of these worlds, there is a set of propositions that completely and accurately describes the given world, or is true of that world. Many philosophers who don't believe in full-blown possible worlds maintain that these sets of propositions are good enough — i.e., that we can take them to be possible worlds.) Or alternatively, one might think of a possible world as a state of affairs, or as a way things could be. In so doing, one might think of these as constituting a new kind of abstract object, or one might think of them as properties — giant, complex properties that the entire universe may or may not possess. For instance, one might say that the actual universe possesses the property of being such that snow is white and grass is green and San Francisco is north of Los Angeles, and so on.