As the late collector Paul Mellon said in 1967, “There is no intellectual or emotional substitute for the authentic, the original, the unique masterpiece.” And so he became a patron of the arts, stacking up one masterpiece after another, works by the titans of 19th- and 20th-century art. In an exhibition that celebrates Mellon and his wife, “Bunny”—two great collectors—70 of his favorites are on view in Padua, including works by Degas, Delacroix, Monet, Picasso, and van Gogh. —E.C.