"It is true that many mathematicians and physicists do their best work when young. But in Einstein's later work we see something much more extreme than the usual falling off. It is as if Thelonious Monk or John Coltrane turned into an obscure twelve-tone composer. How did the greatest physicist since Newton turn into a failed player of mathematical games? All the biographers ask this question; none gives an answer that seems remotely plausible to me as a working scientist."