 Before proceeding, a word about propositions may be helpful: they are abstract objects that can be either true or false, and are commonly identified with sets of possible worlds. A possible world can be thought of as an abstract representation of how things are or could be (Stalnaker 1987; see also entry on  possible worlds).  The proposition that it rains at time \(t\), for example, is just the set of all worlds where it rains at time \(t\). And this particular proposition is true just in case the actual world happens to be a member of the set of all worlds where it rains at time \(t\). 