We call photography “art” with casual unthinkingness now. In Florence there’s a Selfie Museum just round the corner from the museum containing Michelangelo’s David. Can we really describe both as art? But maybe it’s the wrong question. As EH Gombrich put it: “There really is no such thing as Art. There are only artists.” And the exquisite exhibition at MK Gallery of Saul Leiter’s pictures of postwar New York is a masterclass in demonstrating how a photographer can be an artist.