Russell gave an account of how it is possible to talk meaningfully about things that do not exist. According to his theory of definite descriptions, what a proposition of the form “X is Q” means is: “There is exactly one thing that is X and that one thing is Q,” or, “At least one thing is X and no more than one thing is X and whatever is X is Q.”

Russell’s theory makes it possible to distinguish between the contradiction of “X is Q” and “X is not-Q.” To use Russell’s example: (A) “The King of France is bald” has as its contradictory, (A’) “There is no King of France, or there is more than one King of France, or there is one King of France who is not bald.” But, (B) “The King of France is not-bald” means, according to Russell’s theory, (B’) “There is one King of France, no more and no less, and he is not bald.” And, A’ and B’ do not have the same meaning.