Now (obviously) a sentence’s truth—even when we hold the sentence’s meaning fixed—depends on which world we are considering. “Brown is Prime Minister” is true in the actual world but, since Brown need not have been Prime Minister, there are countless worlds in which “Brown 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.
William G. Lycan, Philosophy of Language: A Contemporary Introduction