Galen was a philosopher, physician, pharmacist and prolific medical writer. He compiled an extensive record of the medical knowledge of his day and added his own observations. He wrote on the structure of organs, but not their uses; the pulse and its association with respiration; the arteries and the movement of blood; and the uses of theriacs. "In treatises such as On Theriac to Piso, On Theriac to Pamphilius, and On Antidotes, Galen identified theriac as a sixty-four-ingredient compound, able to cure any ill known". His work was rediscovered in the 15th century and became the authority on medicine and healing for the next two centuries. His medicine was based on the regulation of the four humors (blood, phlegm, black bile, and yellow bile) and their properties (wet, dry, hot, and cold).