Due to the intricate nature of its subject matter, medicine is always threatened by speculations and disagreements about which among its entities exist, e.g., any specific biological structures, substructures or substances, pathogenic agents, pathophysiological processes, diseases, psychosomatic relationships, therapeutic effects, and other possible and impossible things. To avoid confusion, and to determine what entities an item of medical knowledge presupposes to exist if it is to be true, we need medical ontology. The term "medical ontology" we understand to mean the study that seeks to ascertain what entities exist in the world of medicine, which formal relations hold between them, and whether there are any relatioships between types of medical research and practice, on the one hand; and the new worlds they create, on the other.