Those who intend to use phlebotomy must consider first of all how many states of the body there are that call for evacuation. The next question is, which of these states require evacuation by phlebotomy; for there are many conditions, some of which need some other sort of evacuation, and certainly not bloodletting. The third problem is to decide which patients can stand the evacuation without harm, since sometimes the patient's condition demands phlebotomy, yet he cannot bear it either because of his stage of life, or the season of the year, or the nature of the country, or because of some disorder of the mouth of the belly. (This is often miscalled the stomach; in the interests of brevity, however, I shall nevertheless use the term from now on in the whole of this work). There are also some who cannot bear phlebotomy because of their general habit of body, even though by reason of their disease they need it very much. If we are to draw such distinctions, we must consider each occasion on its merits, just as we do with any other remedy. Our next question concerns the veins that are to be opened, since extensive research has been undertaken to decide whether it makes no difference which vein one chooses to cut, all being equally helpful in all conditions, or whether, as Hippocrates and the most celebrated physicians thought, it makes a great deal of difference whether one opens this one rather than that.