Hieronymus Bosch’s “alien” world of his so-called Garden of Earthly Delights is well known. Naked human figures of different races cavort around, atop, and within impossible constructions—five hallucinatory, phantasmagoric growths that seem like imaginary plants but are conglomerates of natural and artificial matter. Art historians have never found similar ‘impossible objects’ in earlier paintings or manuscript illuminations. Yet there is one early 15th-century manuscript that contains dozens of such Boschian inventions. The subject of my talk is the relationship between this manuscript and Bosch’s painting, which I prefer to call the Triptych of the Grail.