André Weil, sister of Simone Weil, compared the mathematical enterprise to sex: "Every mathematician worthy of the name has experienced, if only rarely, the state of lucid exaltation in which one thought succeeds another as if miraculously. Unlike sexual pleasure, this feeling may last for hours at a time, even for days. Once you have experienced it, you are eager to repeat it but unable to do so at will, unless perhaps by dogged work." For many mathematicians the pleasure of practising mathematics outweighs almost any other mental or physical pleasure. Andrew Granville, an English mathematician working at the university of Georgia, told me: "It's the beauty that justifies things... you just have to do it."