Another point must also be kept in view in diagnosis. Diseased action in the body is often very complex, and the phenomena present may not be all reducible to the results of one form of disease, or a morbid condition of one set of organs; it may, on the contrary, be compounded of the effects of several causes acting together. And not only in such a case are the single effects asso ciated together and intermingled with each other, but the product is a combined effect of the compound cause, in which the direct symptoms of each separate lesion are modified or neutralized by one another. It is, therefore, necessary to distinguish between symptoms uniformly associated with certain conditions and those which are merely accidental. These, again, must be divided into phenomena which, though not essential, are more or less directly connected with the morbid state, and those which are wholly in dependent of it. And having collected all the evidence which the case affords, if it appear sufficient to establish ant hypothesis, we have yet to make sure that no other condition of disease is present that might give a different interpretation to some of the symptoms ; and still more, when it is unsatisfactory or contradic tory, must the examination be careful and extensive in order to discover the causes of this imperfection, and the associations which modify or suppress those symptoms which each would display if acting alone.
In every one of these points of view it is evident how much correct diagnosis must depend on a knowledge of the true nature and history of disease. That alone can suggest trustworthy hy potheses for the explanation of the phenomena, by bringing before the mind the different states which commonly give rise to promi nent symptoms, so that when one fails to fulfil all the requirements of the case, another may be substituted for it ; it teaches which among the phenomena are important and constant in their cha racter, which are unimportant and variable; it also indicates the different diseases which are most likely to be associated together, and shows how they mutually react upon one another. And when we have reasoned to the best of our judgment upon the whole of the premises submitted to our consideration, such know ledge can alone supply a standard of comparison, whence we learn what conclusions have been true or false, as the order of events corresponds to or differs from that which scientific experience teaches us to be their known course and progress.
While thus studying diagnosis, let it not be forgotten that though our first aim be to arrive at a correct conclusion regarding the disease under which the patient is laboring, our ultimate object is to restore health. Therefore, while combining symptoms in our own mind to give unity to the whole, we must ever have regard to anything they may teach us concerning the condition of the patient Thus, for example, in any case which may at first sight be regarded as one of the simplest examples of that state to which the much-abused term of inflammation is applied, however clear the evidence in favor of inflammation of any particular viscus, we must not act upon this knowledge alone, but must take into consideration the signs of strength or weakness, of increased or depressed vitality, which accompany it. This oversight is probably the most prolific source of many a hasty and ill-formed assumption, based on insufficient grounds. The self-evident symp toms alone are considered, other phenomena are too often disre garded, sources of fallacy are overlooked, and a diagnosis is pronounced to which the whole course of the disease is made to bend. Of necessity erroneous hypotheses are admitted in order to reconcile the evident discrepancy between the progress of the case and the supposed nature of the malady. Faith in treatment is shaken, because a false opinion once formed, remedies cannot be employed in a manner conducive to the recovery of the patient In the end, the student becomes a fanciful speculator in place of a sober physician. He finds the aimless impotence of quakery as successful as his own misguided efforts, and follows the fashion of the day in homoeopathy, hydropathy, the abuse of the specu lum, &c., to say nothing of the errors into which some have fallen in the introduction of specific modes of treatment, when their position and their knowledge had given promise of better things.