Medicine considers the human body as to the means by which it is cured and by which it is driven away from health. The knowledge of anything, since all things have causes, is not acquired or complete unless it is known by its causes. Therefore in medicine we ought to know the causes of sickness and health. And because health and sickness and their causes are sometimes manifest, and sometimes hidden and not to be comprehended except by the study of symptoms, we must also study the symptoms of health and disease. Now it is established in the sciences that no knowledge is acquired save through the study of its causes and beginnings, if it has had causes and beginnings; nor completed except by knowledge of its accidents and accompanying essentials. Of these causes there are four kinds: material, efficient, formal, and final.
Material causes, on which health and sickness depend, are--- the affected member, which is the immediate subject, and the humors; and in these are the elements. And these two are subjects that, according to their mixing together, alter. In the composition and alteration of the substance which is thus composed, a certain unity is attained.
Efficient causes are the causes changing and preserving the conditions of the human body; as airs, and what are united with them; and evacuation and retention; and districts and cities, and habitable places, and what are united with them; and changes in age and diversities in it, and in races and arts and manners, and bodily and animate movings and restings, and sleepings and wakings on account of them; and in things which befall the human body when they touch it, and are either in accordance or at variance with nature.
Formal causes are physical constitutions, and combinations and virtues which result from them. Final causes are operations. And in the science of operations lies the science of virtues, as we have set forth. These are the subjects of the doctrine of medicine; whence one inquires concerning the disease and curing of the human body. One ought to attain perfection in this research; namely, how health may be preserved and sickness cured. And the causes of this kind are rules in eating and drinking, and the choice of air, and the measure of exercise and rest; and doctoring with medicines and doctoring with the hands. All this with physicians is according to three species: the well, the sick, and the medium of whom we have spoken.