For 1,600 years Dioscorides (ca. AD 40–80) was regarded as the foremost authority on drugs. He knew mild laxatives and strong purgatives, analgesics for headaches, antiseptics for wounds, emetics to rid one of ingested poisons, chemotherapy agents for cancer treatments, and even oral contraceptives. Why, then, have his works remained obscure in recent centuries? Because of one small oversight (Dioscorides himself thought it was self-evident): he failed to describe his method for organizing drugs by their affinities. This omission led medical authorities to use his materials as a guide to pharmacy while overlooking Dioscorides' most valuable contribution—his empirically derived method for observing and classifying drugs by clinical testing. Dioscorides' De materia medica, a five-volume work, was written in the first century. Here revealed for the first time is the thesis that Dioscorides wrote more than a lengthy guide book. He wrote a great work of science. He had said that he discovered the natural order and would demonstrate it by his arrangement of drugs from plants, minerals, and animals. Until John M. Riddle's pathfinding study, no one saw the genius of his system. Botanists from the eighteenth century often attempted to find his unexplained method by identifying the sequences of his plants according to the Linnean system but, while there are certain patterns, there remained inexplicable incoherencies. However, Dioscorides' natural order as set down in De materia medica was determined by drug affinities as detected by his acute, clinical ability to observe drug reactions in and on the body. So remarkable was his ability to see relationships that, in some cases, he saw what we know to be common chemicals shared by plants of the same and related species and other natural product drugs from animal and mineral sources. Western European and Islamic medicine considered Dioscorides the foremost authority on drugs, just as Hippocrates is regarded as the Father of Medicine. They saw him point the way but only described the end of his finger, despite the fact that in the sixteenth century alone there were over one hundred books published on him. If he had explained what he thought to be self-evident, then science, especially chemistry and medicine, would almost certainly have developed differently. In this culmination of over twenty years of research, Riddle employs modern science and anthropological studies innovatively and cautiously to demonstrate the substance to Dioscorides' authority in medicine.