[AD] Μέθοδος. As to what Erasistratus maintains; in the first place, plain fact contradicts his opinion; because a distemper very seldom happens, unless after some of these occurrences, that have been mentioned. And then it does not follow, that what does not affect one person, may not hurt another; nor what does not affect the same person at one time, may not hurt him at another; for there may be some latent circumstances in a body, either in respect of weakness, or some disorder, which either are not in another, nor were in the same body at a different time; and these, though of themselves not considerable enough to cause a distemper, yet may render a body more liable to other injuries: but if he had been sufficiently skilled in the contemplation of the works of nature (which the physicians with very good reason endeavour to attain) he must have known this also, that nothing happens from one single cause; but that is to be taken for the cause, which seems to have contributed most to the effect. Now it is possible, that what does not move at all while alone, may in conjunction with other things excite a great commotion. Besides, Erasistratus himself, who says, that a fever arises from a transfusion of the blood into the arteries, and that this happens in a plethoric body, can assign no reason, why of two persons equally plethoric, the one should fall into a distemper, and the other be free from all danger, which manifestly happens every day. From hence it appears, that supposing this transfusion to be real, nevertheless, when there is a plethora, it does not happen of itself, but when some one of the conditions above-mentioned goes along with it.