  Another option is to take possible worlds as maximally consistent sets of propositions. R. M. Adams (1974) sketched such a theory. If propositions are abstract objects, then on this theory possible worlds are abstract objects. But there are other options open. Adams suggests that someone might, à la Leibniz, take propositions to be thoughts in the mind of God. But if so, and if God is in time and therefore concrete, then presumably his thoughts also are. And if we assume that sets of spatiotemporally located entities are spatiotemporally located (because they are wherever and whenever their members are), then sets of concrete objects are concrete. Thus sets of thoughts of a concrete deity are concrete.