Book 1
pr
As relates to Erasistratus, in the first place the actual evidence is against his opinion, because seldom does a disease occur unless following upon one of these; secondly, it does not follow that what has done no harm to one patient, or to that same patient upon one occasion, may not harm another patient, or the same one at another time. For it is possible that there are certain underlying conditions in the body, whether related to infirmity, or to an actual affection of some kind, which either are not present in another person, or were not existent in that patient on another occasion, and which of themselves are not enough to constitute a disease, yet they may render the body more liable to other injurious affections. But if Erasistratus had been sufficiently versed in the study of the nature of things, as those practitioners rashly claim themselves to be, he would have known also that nothing is due to one cause alone, but that which is taken to be the cause is that which seems to have had the most influence. Indeed it is possible that when one cause acts alone, it may not disturb, yet when acting in conjunction with other causes it may produce a very great disturbance. Moreover, even Erasistratus himself, who says that fever is produced by blood transfused into the arteries, and that this happens in an over-replete body, failed to discover why, of two equally replete persons, one should lapse into disease, and the other remain free from anything dangerous; and that clearly happens every day. Hence, however true this transfusion, one can learn that it does not occur of itself when there is bodily fullness, but when there is added something else.
