In the 1750s Nils Gissler (1715–1771) natural scientist, Doctor of Medicine, member of the Royal Academy of Sciences, the first practising physician in northern Sweden, became mentally ill. Several years later he wrote down his experiences in an extensive account. What is remarkable is how he perceives the devil to be present not in visions and imagination, but as a most physical being and force, penetrating his whole body and senses. His account conforms neither to the classical symptoms of corporal possession nor to traditional images of the devil. Overcoming the devil’s attacks was a school of learning about God’s grace by experience. Personal first-hand experience is also highly valued by Gissler in his scientific research. The aim is to analyse how Gissler interpreted his afflictions in the light of contemporary medical and theological thought, to consider how the experience of his afflictions may have been influenced by the experience of his natural scientific observations, and to provide some critical reflections on the complexity of experience as an empirical category and analytical concept in historical studies.