 In his essay “Pragmatics” (1968), Richard Montague generalized the concept of a possible world to deal with a number of phenomena, including indexicals. An index combines a possible world with other factors relevant to the truth value of a sentence. To study tensed sentences, for example, one incorporates times into indices. A sentence like “Elwood went to the store,” is true in a world, at a time. A sentence like “I went to the store,” would be true in a world, at a time, for a speaker: roughly, if the speaker went to the store prior to the time in the world.