Mike Jay chronicles the scientists, writers, artists, and philosophers who used drugs to explore consciousness before such experiments became countercultural and criminalized. Jay, a historian of science and medicine, traces a tradition running from Humphry Davy's nitrous oxide parties in 1799 through William James's experiments with nitrous oxide and mescaline researchers at Harvard, showing how self-experimentation with psychoactive substances contributed to the emergence of psychology as a discipline. The book examines figures including Sigmund Freud (who championed cocaine), Aldous Huxley (whose mescaline experiences produced The Doors of Perception), and indigenous practitioners whose traditional knowledge informed Western research. Jay situates drug experimentation within broader questions about the nature of mind: What do altered states reveal about ordinary consciousness? How should we understand the relationship between brain chemistry and subjective experience? He traces how the progressive criminalization of drugs severed a research tradition that had produced genuine insights. The book recovers this history without romanticizing it, acknowledging both the valuable knowledge generated and the hazards that accompanied it. Jay writes as a historian sympathetic to his subjects while maintaining critical distance from their sometimes grandiose claims. The result illuminates a hidden prehistory of consciousness studies.