William G. Lycan

Now (obviously) a sentence__ truth__ven when we hold the sentence__ meaning fixed__epends on which world we are considering. __rown is Prime Minister_ is true in the actual world but, since Brown need not have been Prime Minister, there are countless worlds in which __rown is Prime Minister_ is false: in those worlds, Brown did not succeed Tony Blair, or never went into politics, or never even existed. And in some other worlds, someone else is Prime Minister _ David Cameron, P. F. Strawson, me, Madonna, or Daffy Duck. In still others, there is no such office as Prime Minister, or not even a Britain; and so on and so forth. So a given sentence or proposition varies its truth-value from world to world.