Celsus was not a professional doctor of medicine or a surgeon, but a practical layman whose "On Medicine," written in a clear and neat style, for lay readers, is partly a result of his medical treatment of his household (slaves included) and partly a presentation of information gained from many Greek authorities. From no other source can we learn so much of the condition of medical science up to his own time. Book I of "De Medicina": after an excellent survey of Greek schools (Dogmatic, Methodic, Empiric) of medicine come sensible dietetics or health preservation which will always be applicable. Book II: deals with prognosis, diagnosis of symptoms (which he stresses strongly), and general therapeutics. Book III: internal ailments: fevers and general diseases. Book IV: local bodily diseases.