Certain moments of the mind have a special quality of well-being. A mathematician friend of mine remarked that his daughter, aged eight, had just stumbled without his teaching onto the fact that some numbers are prime numbers--those, like 11 or 19 or 83 or 1,023, that cannot be divided by any other integer (except, trivially, by 1). "She called them 'unfair numbers'," he said. "And when I asked her why they were unfair, she told me, "Because there's no way to share them out evenly."" What delighted him most was not her charming turn of phrase nor her equitable turn of mind (17 peppermints to give to her friends?) but--as a mathematician--the knowledge that the child had experienced a moment of pure scientific perception. She had discovered for herself something of the way things are. The satisfaction of such a moment at its most intense--and this is what ought to be meant, after all, by the tarnished phrase "the moment of truth"--is not easy to describe. It partakes at once of exhilaration and tranquillity. It is luminously clear. It is beautiful. The clarity of the moment of discovery, the beauty of what in that moment is seen to be true about the world, is the most fundamental attraction that draws scientists on.