information:classification, information:extraction
Localization of function

One of Galen's major works, On the Doctrines of Hippocrates and Plato, sought to demonstrate the unity of the two subjects and their views. Using their theories, combined with Aristotle's, Galen developed a tripartite soul consisting of similar aspects. He used the same terms as Plato, referring to the three parts as rational, spiritual, and appetitive. Each corresponded to a localized area of the body. The rational soul was in the brain, the spiritual soul was in the heart, and the appetitive soul was in the liver. Galen was the first scientist and philosopher to assign specific parts of the soul to locations in the body because of his extensive background in medicine. This idea is now referred to as localization of function. Galen's assignments were revolutionary for the time period, which set the precedent for future localization theories.

Galen believed each part of this tripartite soul controlled specific functions within the body and that the soul, as a whole, contributed to the health of the body, strengthening the "natural functioning capacity of the organ or organs in question". The rational soul controlled higher level cognitive functioning in an organism, for example, making choices or perceiving the world and sending those signals to the brain. He also listed "imagination, memory, recollection, knowledge, thought, consideration, voluntary motion, and sensation" as being found within the rational soul. The functions of "growing or being alive" resided in the spirited soul. The spirited soul also contained our passions, such as anger. These passions were considered to be even stronger than regular emotions, and, as a consequence, more dangerous. The third part of the soul, or the appetitive spirit, controlled the living forces in our body, most importantly blood. The appetitive spirit also regulated the pleasures of the body and was moved by feelings of enjoyment. This third part of the soul is the animalistic, or more natural, side of the soul; it deals with the natural urges of the body and survival instincts. Galen proposed that when the soul is moved by too much enjoyment, it reaches states of "incontinence" and "licentiousness", the inability to willfully cease enjoyment, which was a negative consequence of too much pleasure.

In order to unite his theories about the soul and how it operated within the body, he adapted the theory of the pneuma, which he used to explain how the soul operated within its assigned organs, and how those organs, in turn, interacted together. Galen then distinguished the vital pneuma, in the arterial system, from the psychic pneuma, in the brain and nervous system. Galen placed the vital pneuma in the heart and the psychic pneuma (spiritus animalis) within the brain. He conducted many anatomical studies on animals, most famously an ox, to study the transition from vital to psychic pneuma. Although highly criticized for comparing animal anatomy to human anatomy, Galen was convinced that his knowledge was abundant enough in both anatomies to base one on the other. In his treatise On the usefulness of the parts of the body, Galen argued that the perfect suitability of each part of the body to its function indicated the role of an intelligent creator. His creationism was anticipated by the anatomical examples of Socrates and Empedocles.