A negative history of science rests on the assumption that scientific progress, in its plurality and historical complexity, can be grasped precisely where it becomes fragile: in the handling of errors and ignorance, in the warding-off of un-knowledge, or in the disputes over which authority may count as scientific and what is excluded as “pseudoscience.” These oppositions gain a particular sharpness where the boundary between invisibility and visibility shifts — as in occultism, spiritualism and the esoteric margins of modern science.