Art is regarded as part of a wide aesthetic world, not sealed in a vacuum, so Robert Gober’s “Untitled Leg” spurs associations not just to Duchamp’s 1917 readymade urinal “Fountain,” Meret Oppenheim’s 1936 “Object” and Duane Hanson’s 1970s Madame Tussaud-like sculptures but also to an Alfred Hitchcock movie and Oz’s Wicked Witch of the West.