On Medicine (De Re Medica)

Since, however, of the three parts of medicine, that which heals diseases is as difficult as it is famous, one must speak of it before all else. And because the first disagreement within it is that some contend that only the knowledge of experiments is necessary for themselves, while others propose that usage is not powerful enough unless the reason for bodies and natural things is discovered, it must be indicated what is most strongly argued on both sides, so that our own opinion may be inserted more easily.

Therefore, those who profess rational medicine propose that these things are necessary: knowledge of the hidden causes that contain diseases, then of the evident ones, after these of natural actions as well, and lastly of the internal parts.

They call ABDITÆ hidden those causes in which it is sought from what principles our bodies are composed, and what makes for health and what for adverse health. For they do not believe that someone can know how it is fitting to cure diseases if he is ignorant of where they come from; nor that there is any doubt that a different treatment is needed if something either exceeding or deficient from the four principles creates adverse health, as some of the professors of wisdom have said; or another if all the vice is in the fluids, as seemed true to Herophilus; or another if in the breath, as to Hippocrates; or another if blood is transfused into those vessels which are adapted for the breath, and excites an inflammation—which the Greeks name phlegmone inflammation—and that inflammation causes such movement as is in a fever, as it pleased Erasistratus; or another if flowing particles subsisting through invisible pores close the path...