One with diarrhoea, and one with rheumatism, while three may escape without any injury. Not less remarkable is the uncertainty in regard to the action of remedies. One case appears to yield with readiness to the remedies that are employed; on another, which we have every reason to believe to be of the same nature, no effect is produced in arresting its fatal progress; while a third, which threatened to be equally formidable, appears to cease without the operation of any remedy at all.

With these complicated sources of uncertainty, which meet us at every step in our medical inquiries, it is almost unnecessary to contrast the perfect uniformity of phenomena, on a confidence in which we proceed in other departments of science. When we mix together pieces of zinc, sulphuric acid, and water, we pronounce with perfect confidence that the water will be decomposed, hydrogen evolved, the metal oxidated, the oxyd dissolved, and sulphate of zinc produced; we pronounce with equal confidence on all the properties, mechanical and chymical, of the new compound which is thus to be formed; and in no case have we the smallest doubt of the exact occurrence of every step in this complicated process. With what different feelings we contemplate, in its commencement, a case of dangerous internal disease, its probable progress and termination, and the effect which our remedies are likely to produce in arresting it,-those best can tell who have most experienced them.

The certainty of a science, as was formerly stated, depends upon two circumstances; namely, the facility with which we ascertain the true relations and tendencies of things, or trace effects to their true causes, and causes to their true effects; and the confidence with which we rely on the actions, dependent on these relations, continuing to occur in all cases with perfect uniformity. This confi

dence we easily attain in those sciences in which we have to deal only with inanimate matter. We do so by means of experiments, in which, by placing the substances in various circumstances towards each other, we come to ascertain their true tendencies with perfect certainty, and to separate them from the influence of all associations which are only casual and incidental. Having thus discovered their tendencies or actions, we rely with confidence on these continuing to be uniform; and should we in any instance be disappointed in the action which we wish to produce, we are able to trace the cause by which the expected result has been prevented, and to obviate the effect of its interference.

In both these respects we find in medicine a degree of uncertainty which marks a striking distinction between it and the purely physical sciences.

I. There is great difficulty in medicine in tracing effects to their true causes, and causes to their true effects. This difficulty has already been illustrated by the same cause appearing to produce in different instances different diseases, or no disease at all; and by a disease seeming to subside under the use of a remedy which, in a similar case, fails to produce the smallest benefit. When we find our researches thus encumbered with uncertainty, we cannot, as in other sciences, clear them from the influence of casual relations by means of direct experiment; but are obliged to trust chiefly to the slow course of observation, as the relations happen to be presented to us. Hence just conclusions are arrived at slowly, and we may be obliged to go on through a long course of observations, before we arrive at any results which we feel to be worthy of confidence. Hence also arises the great temptation to grasp at partial and premature conclusions, from which medical science has suffered so much injury. For when such conclusions are brought forward with confidence,

as long a course of observation may be required for exposing their fallacy as might have been sufficient for ascertaining the truth. In this respect we see the remarkable difference between medicine and the purely physical sciences; as, in the latter, a single experiment may often be sufficient to overturn the most plausible hypothesis, or to establish one which has been proposed only in conjecture.

II. Even after we have ascertained the true relations and tendencies of things, we are constantly liable to disappointment in medicine, when we endeavour to produce certain results by bringing these tendencies into action. This arises from the silent operation of a new order of causes, by which the phenomena of disease are varied and modified; and by which the action of external agents is aided, modified, or counteracted in a manner which altogether eludes our researches. The causes which thus operate are certain powers in the living body itself, the action of which is entirely beyond our control; and others arising out of the peculiarities of age, sex, temperament of body and mind, and mental emotions; constituting a class of agents of a most powerful kind, of which it is impossible to estimate the combined operation. It is farther to be kept in view, that these various agents may be acting together, or in opposition to each other, or under a variety of combinations; and that, in reference to our attempts to act upon the body by remedies, they may be operating in concert with, or in opposition to these attempts. Hence arises a most extensive source of uncertainty in all our investigations, of which it is impossible to calculate the effect, or the extent. Hence also arises that apparent want of uniformity in the phenomena of disease, by which we are so much impeded in our researches; and that want of uniformity in the action of remedies, by which our efforts in medicine are so often disappointed.

III. Another source of uncertainty in the practical art of medicine is the difficulty which we find in applying to new cases the knowledge which we have acquired from observation. This application is made upon the principle either of experience or analogy. We are said to proceed upon experience when the circumstances in the new case are the same as in those cases from which our knowledge was derived. When the circumstances are not the same, but similar, we proceed upon analogy; and our confidence in the result is weaker than when we proceed upon experience. The more numerous the points of resemblance are, the greater is our confidence, because it approaches the more nearly to that which we derive from experience; and the fewer the points of resemblance, our confidence is more and more diminished. When, in the practice of medicine, we apply to new cases the knowledge acquired from others which we believe to have been of the same nature, the difficulties are so great, that it is doubtful whether in any case we can properly be said to act upon experience, as we do in other departments of science. For we have not the means of determining with certainty, that the condition of the disease, the habit of the patient, and all the circumstances which enter into the character of the affection, are in any two cases precisely the same: and if they differ in any one particular, we cannot be said to act from experience, but only from analogy. The difficulties and sources of uncertainty which meet us at every stage of such investigations are, in fact, so great and numerous, that those who have had the most extensive opportunities of observation will be the first to acknowledge that our pretended experience must, in general, sink into analogy, and even our analogy too often into conjecture.

In a science encumbered with so many difficulties, and encompassed by so many sources of error, it is obvious what cause we have for proceeding with the utmost caution, and for advancing from step to step

with the greatest circumspection. In attempting a slight outline of a subject so extensive and so important, I shall confine myself to a few leading rules of a strictly practical nature.

The objects to be kept in view in all our investigations appear to be the following :—

I. To acquire an extensive collection of well-authenticated facts.

II. To arrange, classify, combine, or separate these facts.

III. To trace among the facts, sequences, or relations, particularly the relation of cause and effect.

IV. From an extensive collection of facts to deduce general facts or general principles.

SECTION I.

OF THE ACQUISITION AND RECEPTION OF FACTS.

THE foundation of all knowledge must be a careful and extensive acquisition of facts; and the first duty of an inquirer in any department of science is to bind himself down to such a patient accumulation, bewaring of all premature attempts to combine or generalize them.

In the acquisition of facts, we depend partly on our own observation, and partly on the testimony of others. The former source is necessarily limited in extent, but it is that in which we have the greater confidence; for, in receiving facts on the testimony of others, we require to be satisfied, not only of the veracity of the narrators. but also of their habits as