With imperiously graceful gestures, Jackson Pollock flings pigments through the air. His technique is dramatic, it is astonishing, but it is not entirely original. Hoping to exploit the raw force of the unconscious, the Surrealists had poured and splashed their paints. There are dripped passages in certain canvases by Hans Hofmann, the European maestro who introduced a generation of New Yorkers to the traditions of the avant-garde. David Alfaro Siqueiros and Thomas Hart Benton are among those who dispensed with brushes before Pollock did. Their experiments are nearly forgotten. Pollock's paint-slinging is remembered because it generated a power that overwhelms understanding. Critics and historians describe him as a wrenchingly expressive artist, a major formal innovator, a brilliant stylist. Pollock was all that, as were others. What is unique about him has been elusive.