The simplicity of disease is thus destroyed by sympathy, even when all the organs, except the one which is attacked, are in a healthy state at the time of the attack. And when they are already in an unhealthy, unnatural condition from previous disease, the complication is rendered still greater. _Chronic_[2] cases especially are often so complex from this cause, that it requires the most discriminating acumen to unravel their history, and make out the starting point of the disease. Often it is impossible to discover any such starting point; and sometimes there is none, but there are several different diseases in different organs, all affecting each other through sympathy, and presenting together a confused and changing medley of symptoms. In such cases, the manifestations of diseased action are at one time most prominent in one organ, and at another time in another. These variations in the phase of the disease are often so unaccountable, as to seem capricious, and they always embarrass the physician, as he attempts to determine the effect of his remedies, and to proportion them to the importance of the symptoms, as they show themselves in the various organs. It would sometimes almost seem, that a tricksy little spirit were playing its pranks among the organs, now here and now there, eluding his search, and escaping his grasp. In some cases, disease will leave the organ in which it seems to be obstinately fixed, and appear in full force in some other organ, which has been up to that time only sympathetically affected. This is more apt to occur in children, because the sympathies are more lively in them than they are in adults. Such changes, taking place often without any obvious cause, and so suddenly, and sometimes, we may add, so secretly, you can readily see, must tend to make our knowledge of disease, and of the effect of remedies, confused and uncertain. 2. The influence of unseen or secret causes, is another source of uncertainty in medicine. The fact, that some causes, whose nature and extent cannot be appreciated, are at work modifying disease, and the effects of remedies, constantly forces itself upon the attention of the practitioner. The causes of disease, and of the changes that occur during its progress, are much more concealed from our view than is generally supposed. Patients are fond of fixing upon something to which they can attribute their sickness; but in the great majority of cases, the conclusion which they adopt with so much confidence is a mere supposition, and does not rest upon any substantial proofs. Even in the case of a common cold, you will find that the reasons given for believing that this or that cause produced it, often will not bear a strict examination, according to the acknowledged rules of evidence. Ordinarily some exposure is looked upon as being without a doubt the cause, when it may have been only one of the causes, or may even have had no agency at all in producing the result.