Gustave Courbet’s The Origin of the World – his clinically voyeuristic 1866 oil painting of a woman shown literally and solely as a sex object, with all distractions, such as her face, ruthlessly removed – hangs in the Musée d’Orsay in Paris, opposite his far larger canvas A Burial at Ornans (1849–50). It is an incredibly powerful juxtaposition, an unforgettable double act. The small, yet white and bright and unavoidably shocking Origin looks across a shadowy space at the huge, dark, funeral scene, with its enigmatic rural faces gathered around the black void of a grave. If you could ask Courbet what he believed in, this display makes it plain…