As that branch of medicine, which respects the cure of diseases, is the noblest, as well as the most difficult of the three, we shall first treat of that part. And because in this the chief dispute is, that some alledge an acquaintance with experiments to be only requisite, while others affirm experience alone to be insufficient, without a thorough knowledge of the constitution of bodies, and what naturally happens to them; it will be proper to recite the principal arguments on both sides, that we may the more easily deliver our own opinion upon the question. Those then, who declare for a theory in medicine, look upon the following things as necessary: the knowledge of the occult and constituent causes of distempers; next, of the evident ones; then, of the natural actions; and, lastly, of the internal parts. They call these causes occult, in which we inquire of what principles our bodies are composed, what constitutes health, and what sickness. For they hold it impossible that any one should know how to cure diseases, if he be ignorant of the causes, whence they proceed; and that it is not to be doubted, but one method of cure is required, if the redundancy or deficiency in any of the four principles(3) be the cause of diseases, as some philosophers have affirmed; another, if the fault lie wholly in the humours, as Herophilus thought; another, if in the inspired air, as Hippocrates believed; another, if the blood be transfused into those vessels(4), which are designed only for air, and occasion an inflammation, which the Greeks call phlegmone[AB], and that inflammation cause such a commotion as we observe in a fever, which was the opinion of Erasistratus; another, if the corpuscles passing through the invisible pores should stop, and obstruct the passage, as Asclepiades maintained: that he will proceed in the proper method of curing a disease, who is not deceived in its original cause. Nor do they deny experience to be necessary, but affirm, it cannot be obtained without some theory; for that the more ancient practitioners did not prescribe any thing, at hazard, for the sick, but considered what was most suitable, and examined that by experience, to which they had before been led by some conjecture. That it is of no moment _in this argument_, whether most remedies were discovered by experiment, provided they were at first applied with some rational view: and that this holds in many cases; but new kinds of distempers often occur, in which practice has hitherto given no light; so that it is necessary to observe whence they arose; without which no mortal can find out, why he should make use of one thing, rather than another. And for these reasons they investigate the occult causes. They term those causes evident, in which they inquire, whether the beginning of the distemper was occasioned by heat or cold, fasting or surfeit, and the like. For they say, he will be able to oppose the first appearances, who is not ignorant of their rise.