The stone of madness, also called stone of folly, was a hypothetical stone in a patient's head, thought to be the cause of madness, idiocy or dementia. From the 15th century onwards, removing the stone by trepanation was proposed as a remedy.[1][2] This procedure is demonstrated in the painting The Extraction of the Stone of Madness by Hieronymus Bosch.[3]
Gallery
Quentin Massys, An Allegory of Folly (early 16th century). The fool has a "stone of folly" in his forehead.
Pieter Huys, A surgeon extracting the stone of folly