In room 46 of the London National Gallery, two portraits hang, one by classical painter Jean-Auguste-Dominique Ingres, the other by Pablo Picasso. Ingres’s portrait is of society beauty, Madame Moitessier (1856). Picasso’s portrait, several metres away, is of his mistress, Marie-Thérèse Walter, entitled Woman with a Book (1932).   You may wonder why these works have been given…