A perception of the errors arising from this cause has led to one of an opposite tendency, which teaches that the general con dition of the patient must be alone considered, and that the name or nature of the disease is a matter of secondary importance. In the hands of a man of sound judgment and accurate perception, such a course is probably less injurious to the patient than a false conclusion formed on insufficient premises. Its peculiar evil consists in its leaving the student without a scheme or proposition, around which to collect and arrange the multitude of distinct and isolated facts which any case in the wards brings before him. Few minds, even those of the highest order, are able to divest them selves wholly of hypothesis in considering tiny series of facts; and the more untrained the mind is, the more readily does it frame such hypotheses for the purpose of explaining them. By the term explaining- we only mean the referring the phenomena to some more general principle, which seems to stand to them in the relation of cause to effect, and includes in itself the whole or any number of the facts under consideration, as its necessary or com mon results or consequence& It is impossible to avoid affording such explanations to the student who is acquiring the priiiciples of medicine, and it seems unwise to discard them in the wards of the hospital, where every case ought to be only an example of the doctrines t,aught in the schools; and if the teacher do not supply the hypothesis, which in any given case seems to him to afford the true solution of the phenomena, the student will naturally frame one for himself, and that probably an erroneous one. At the same time it must be admitted by those who themselves are the most expert in the practice of diagnosis, that the time and the opportunity are not commonly afforded to give these explana tions at the bedside of the patient; and clinical lectures can only take up the more prominent class, or the more remarkable indi vidual cases which at the time happen to be in the hospital, and much of the remainder is lost for the purposes of instruction.
My object in the following pages is not to supersede the teach ing of the clinical physician, but to meet this necessary imperfec tion by pointing out to the student how he may best frame a true scheme for himself, and still more to aid him in learning the lesson he is daily taught, by rendering familiar to him the principles on which the physician himself forms his opinion.. It cannot need any demonstration to show that one who has thus studied will, when himself called upon to prescribe, all the more readily seize on the true form of the disease, and the exact relation it holds to the vital condition of the patient.
In carrying out this intention it would be equally valueless to give a mere enumeration of symptoms, or to classify the excep tions which experience has taught myself and others to look for, and the errors into which we are liable to fall. My purpose is to elucidate the principles as well as the practice of their interpreta tion, so that whatever be their variety or perplexity, philosophical conclusions may be drawn from their presence, avoiding unwar rantable inferences, and at least guiding the mind in a right direc tion, if no satisfactory solution of any individual case can be arrived at.
All true diagnosis is ultimately based upon inductions separately framed out of clinical and pathological investigations and experi ments. By careful and repeated observation, we have succeeded, with every appearance of truth, in associating certain phenomena observed during life with particular lesions found after death; and these form the first step in our progress. Sound principles have advanced exactly in proportion to the number and the accuracy of these conclusions, because there are many conditions which we are not yet, and perhaps never shall be, able to associate with any appreciable change in structure; and to them we must apply by inference the truths which have been taught in other instances by direct observation. In so far as we are able correctly to interpret symptoms, and to trace out in connection with them a real change of structure or of function which affords an adequate explanation of their presence, in so far are we prepared to form a correct diagnosis. It is not the province of this branch of study to eluci date the modus operandi of the change; but, assuming these principles as true, our especial work is to learn to group symp toms together, and to analyze them separately in such a manner that we may be able to apply to them a scheme already supplied to our hand, which shall in some way account for their existence. It is by the nature of this assumption that rational medicine is distinguished from empiricism. The latter equally seeks to group symptoms together, and to assign to each group the most suitable remedies; but the theory or scheme which it furnishes is not based on scientific principles. In the application of the theory to the case under observation, the two are exactly analogous. A comparison is to be instituted between the probable results of the supposed malady and those presented by the particular case, and their correspondence serves for the verification of the hypothesis. In short, it is the deductive process of reasoning applied to the elucidation of morbid phenomena. We gather together in the best ra anner we can the fragmentary evidence of sympt,oms, and we apply to it the knownlaws of c,ausation taught by the theory of dise,ase.
The correctness with which this process is performed depends on a variety of circumstances. In the first place, it will be greatly influenced by the amount of evidence. This evidence has to be sought, and therefore much wW. depend on the manner in which the investigation is conducted. Without method, some portion of it is sure to be overlooked or forgotten ; with a bad method, the information presents itself in such a form as inakes the in ference of truth a matter of difficulty. The plan adopted in this volume is one which, on close consideration of the subject, has most commended itself to my own mind; but each person will probably be disposed t,o modify it so as to suit his own habits of thought.