Self-referential sentences are not simply an idle indulgence; the famous logician Kurt Gödel used a mathematical formulation of the Liar's Paradox to draw very profound conclusions about the power of mathematics. (We shall be saying more about Gödel's results later.) For more on self-referntial sentences, see Metamagical Themas: Questing for the Essence of Mind and Pattern by Douglas R. Hofstadter (Bantam Books, New York 1986)