By means of compositional devices and a masterful use of Greek language that the author adapts to the contents, Galen achieves an accurate and didactic exposition, which presents the vivacity of oral speech. Dealing with a humoral unbalance that is on the basis of many local and general affections —inflammation and fevers among them—, this small treatise also includes key notions of Anatomy and Physiology. On those grounds it became a bestseller in the Middle Ages and the Renaissance (translations into Syrian, Arabic, Latin —8 versions— and Hebrew; 14 commentaries from 1290 to 1567). This first critical edition presents a continuous text—no chapters— and points to the internal structure created by the author in order to make his text readily accessible —and unforgettable. A thorough Introduction —the textual transmission, the fundamental topics—, is coupled by an ample Commentary that deals with alteration and pain, qualities and humours, no less than with scribal errors and grammatical explanations.