Kendrick Lamar’s recent single ‘The Heart Part 5’ samples ‘I Want You’, Marvin Gaye’s torch song for Janis Hunter. The 1970s saw Gaye whipsaw from socially conscious poet (What’s Going On, 1971) to lover man (Let’s Get It On, 1973) to lovelorn Janis-stan (I Want You, 1976). (Maybe he was all of these things at many times in his life, as David Ritz’s biography suggests, but his public-facing self seemed to transform with each new record.)