We were looking at a reproduction of Jackson Pollock's breakthrough work, Mural , an 8-by 20-foot canvas bursting with physical energy that, in 1943, was unlike anything seen before. The critic Clement Greenberg, Pollock's principal champion, said he took one look at the painting and realized that "Jackson was the greatest painter this country has produced." A Museum of Modern Art curator, the late Kirk Varnedoe, said Mural established Jackson Pollock as the world's premier modern painter. I was researching a book about Pollock's lifelong relationship with his mentor, Thomas Hart Benton, the famed regionalist and muralist, when I sat puzzling over a reproduction of Mural after breakfast one morning with Marianne, herself an art historian. She suddenly said she could make out the letters S-O-N in blackish paint in the upper right area of the mural. Then she realized JACKSON ran across the entire top. And finally she saw POLLOCK below that. The characters are unorthodox, even ambiguous, and largely hidden. But, she pointed out, it could hardly be random coincidence to find just those letters in that sequence. I was flabbergasted. It's not every day that you see something new in one of the 20th century's most important artworks. I'm now convinced that Pollock wrote his name in large letters on the canvas -- indeed, arranged the whole painting around his name. As far as I can tell, no one has previously made this assertion. Nor is there evidence that Pollock himself, who was loath to talk about