Cave paintings of our prehistoric ancestors, elaborate ritual dances of preliterate tribesmen, long lines at the movies, earnest scribbles of the three-year-old next door--evidence of human preoccupation with art is everywhere, and it is overwhelming. But unlike other human universals--language, tool use, the family--art makes no material contribution to mankind's survival. What impels the artist to the lonely effort at self-expression? What moves...