Avicenna was a physician, naturalist, and writer. He studied philosophy, logic, medicine and at the age of 16 took up medical practice. He produced possibly 100 treatises of varying lengths. His work Canon of Medicine was widely read for several centuries and was the recognized guide of medical study in European Universities from the 12th to the 17th Centuries. Along with Aristotle and Pliny he was continuously quoted by 13th Century writers.