Galen transmitted Hippocratic medicine all the way to the Renaissance. His On the Elements According to Hippocrates describes the philosopher's system of four bodily humours, blood, yellow bile, black bile and phlegm, which were identified with the four classical elements, and in turn with the seasons. He created his own theories from those principles, and much of Galen's work can be seen as building on the Hippocratic theories of the body, rather than being purely innovative. In turn, he mainly ignored Latin writings of Celsus, but accepted that the ancient works of Asclepiades had sound theory.
Amongst Galen's own major works is a seventeen-volume On the Usefulness of the Parts of the Human Body. He also wrote about philosophy and philology, as well as extensively writing on anatomy. His collected works total twenty-two volumes, and he wrote a line a day for most of his life.
Galen's own theories, in accord with Plato's, emphasized purposeful creation by a single Creator ("Nature" - Greek phusis) - a major reason why later Christian and Muslim scholars could accept his views.
His fundamental principle of life was pneuma (air, breath) that later writers connected with the soul. These writings on philosophy were a product of Galen's well rounded education, and throughout his life Galen was keen to emphasise the philosophical element to medicine. Pneuma physicon (animal spirit) in the brain took care of movement, perception, and senses. Pneuma zoticon (vital spirit) in the heart controlled blood and body temperature. "Natural spirit" in the liver handled nutrition and metabolism. However, he did not agree with the Pneumatist theory that air passed through the veins rather than blood.
Galen expanded his knowledge partly by experimenting with live animals. One of his methods was to publicly dissect a living pig, cutting its nerve bundles one at...