The poet Philip Larkin famously declared that the English discovered sex in 1963, but a new show at Tate Britain proposes the far earlier date of 1782, the year curious Londoners flocked to the Royal Academy's summer exhibition to look in amazement, confusion and excitement at Henry Fuseli's painting "The Nightmare."

There, amid the usual worthy portraits and landscapes, Fuseli's oil displayed the prostrate body of a sleeping maiden, with a depraved-looking ogre or incubus sitting on her chest and the head of a blind horse protruding menacingly through red velvet curtains. What could it mean?