Seventy this month, Bob Dylan continues releasing albums and touring the world. He isn’t the only pensionable rocker who goes on working, of course. Paul McCartney still tours, and the Rolling Stones occasionally fill big halls. But none of them shares Dylan’s devotion to his work: since his sixtieth birthday, he has averaged 100 or so gigs a year. Dylan is alone, too, in constantly reinterpreting his back catalogue. McCartney’s arrangements of his songs haven’t changed since the day they were recorded. Dylan’s renditions of even his most famous numbers differ on an almost daily basis. His songs have meant different things to him at different stages of his life.