The central principle of belief of Hippocrates and the Dogmatists was that health depended on the proper proportion and action in the body of the four elements, earth, water, air, and fire, and the four cardinal humours, blood, phlegm, yellow bile and black bile. The due combination of these was known as _crasis_, and existed in health. If a disease were progressing favourably these humours became changed and combined (coction), preparatory to the expulsion of the morbid matter (crisis), which took place at definite periods known as critical days. Hippocrates also held the theory of fluxions, which were conditions in the nature of congestion, as it would now be understood. In his time public opinion condemned dissection of the human body, but it is certain that dissections were performed by Hippocrates to a limited extent. He did not know the difference between the arteries and the veins, and nerves and ligaments and various membranes were all thought to have analogous functions, but his writings display a correct knowledge of the anatomy of certain parts of the body such as the joints and the brain. This defective knowledge of anatomy gave rise to fanciful views on physiology, which, among much that is admirable, disfigure the Hippocratic writings. The belief that almost all medical and surgical knowledge is modern, though flattering to our self-complacency, is disturbed by the study of the state of knowledge in the time of Hippocrates. To him we are indebted for the classification of diseases into sporadic, epidemic, and endemic, and he also separated acute from chronic diseases. He divided the causes of disease into two classes: general, such as climate, water and sanitation; and personal, such as improper food and neglect of exercise. He based his conclusions on the observation of appearances, and in this way began a new era. He was so perfect in the observation of external signs of disease that he has never in this respect been excelled. The state of the face, eyes, tongue, voice, hearing, abdomen, sleep, breathing, excretions, posture of the body, and so on, all aided him in diagnosis and prognosis, and to the latter he paid special attention, saying that "the best physician is the one who is able to establish a prognosis, penetrating and exposing first of all, at the bedside, the present, the past, and the future of his patients, and adding what they omit in their statements. He gains their confidence, and being convinced of his superiority of knowledge they do not hesitate to commit themselves entirely into his hands. He can treat, also, so much better their present condition in proportion as he shall be able from it to foresee the future."