The imprecise status of Psychiatry may be due to Cartesian dualism and the mind / body assumption. We can clarify this status resorting to an alternative ontology. Popper's Worlds 1, 2 and 3, and the corresponding concepts of matter, energy and information, from Von Bertalanffy and his General Systems Theory, are candidates for a new ontology. Applying them to the history and present structure of Western Medicine, we can assume that General Surgery is the branch of medicine which deals with bodily matter, Internal Medicine deals with energy and Psychiatry is the branch which deals with information. The respective basic disciplines are Anatomy, Physiology and Psychology. None of them is a medical specialty, because specialties, such as Neurology, Urology and Cardiology, deal with a sub-system of the organism and may have a surgical, medical or even psychiatric emphasis. In the same way that the body / mind problem seemed important for Cartesian dualism, this triadic structure of Medicine may also inform a new ontology. The recent history of Philosophy may be interpreted as an attempt of looking for it.