His main theory was that every part of man's body is there for a special and particular purpose, and that it is possible to understand that purpose fully by constant study, and that an understanding of the body will bring an understanding of God's plan in creating it, and so a better understanding of the purpose of all nature.
Galen had learned anatomy on a human skeleton, but for physiology (which means the science of the processes of life in animals and plants) he had to rely on the bodies of cattle, monkeys and pigs. He tried to explain respiration (breathing) and also how the blood came into being and what its function was, and it was here that he made a major, though understandable, mistake, which was accepted as the truth until well into the 17th century. He thought that food when eaten turned into a thick whitish fluid which was carried to the liver where the best part of it was turned into blood, the humors, and the 'natural' or 'vegetable spirits' whose function was to control all the normal bodily processes of growth and nutrition.
Vegetable spirits were converted into 'vital spirits' in the heart, and these, Galen believed, kept the body alive. The vital spirits reached the brain and were purified there to become 'animal' (or soul) spirits which, by flowing along the nerves which were thought by Galen to be hollow, allowed the whole body to obey the brain in movement and emotion.
Galen's theories, which were not necessarily his own invention, but were probably derived from all such earlier medical thought as he considered worthy of note, were never seriously questioned by doctors until well into the 17th century. This happened because for a century after his death Galen was still the accepted authority whom no one dared to question. After that, by AD 400, the Christian Church began to oppose both ancient and new medical thought; the first because of its pagan, or non-Christian origins, the second in case it should cause men to question their faith or to treat any form of God's creation - even dead bodies - disrespectfully, by dissecting them to find out more about how they worked.
Therefore, for about 800 years medical knowledge slowly declined, although the monastic libraries preserved many ancient medical writings.