Christian Harting, left, and Geza Rohrig in a scene from “Son of Saul.”Credit Sony Pictures Classics, via Associated Press I. Art is often the subject of philosophy. But every now and then, a work of art — something other than a lecture or words on a page — can function as philosophy. “Son of Saul,” a film set in Auschwitz-Birkenau during the Holocaust, is such a work of art. It engages with a pro