and who have had sufficient opportunities of acquiring them.

II. That the statement include a full and fair view of all the facts which ought to be taken into the investigation; that none of them be disguised, or modified so as to be made to bear upon a particular doctrine; and that no essential facts be wanting.

III. That the statement do not include facts which are trivial, incidental, or foreign to the subject.

IV. That we do not receive as facts statements which are not facts, but opinions or general assumptions.

SECTION II.

OF ARRANGING, COMBINING, AND SEPARATING FACTS.

THE precautions now suggested appear to be those which it is necessary to keep in view in making a collection of facts respecting any subject under investigation. Our next step is to arrange the facts according to the characters in which they agree; to separate from the mass those which appear to be only fortuitous or occasional concomitants; and to place by themselves those which we have reason to consider as a uniform and legitimate series or sequence. This is the first step towards tracing the relations of the facts; and in every investigation it is a process of the utmost consequence. In the other departments of physical science this object is accomplished by means of experiments. These are so contrived as to bear distinctly upon particular points; and by the result of them we are enabled to separate

associations which are incidental from those which are uniform; or, in other words, to ascertain what number of the circumstances which we find associated in a particular series are really connected with the result which follows. In medicine this is a process of greater difficulty, because we are obliged to trust to the slower course of minute and long-continued observation.

The rules to be observed under this head are essential to every department of medical inquiry; but, perhaps, they are peculiarly important in our observations respecting the phenomena of disease. By this, we mean such an acquaintance with the symptoms which characterise particular diseases, and the morbid appearances in the cases which are fatal, as shall enable us to trace the relation between the symptoms and the nature and seat of the disease. A full collection of uniform and essential facts on these subjects, cleared as far as we are able from all incidental combinations, is the only true foundation of medical science; and every system, however ingenious, which rests upon any other, can be nothing better than hypothesis and conjecture. It is an essential but difficult part of medical investigation, and one which we must conduct with much patience, without allowing ourselves to be seduced by theory or system from the path of rigid observation. In prosecuting it we must be cautious in considering our conclusions as perfect, but make it our constant study by further observation to clear them more and more from every source of error.

Whatever leads the mind from the importance and the difficulty of this investigation is injurious to medical science. The error to be chiefly avoided is a fondness for system; and I must confess my suspicion that, in this respect, a zeal for nosology has been unfavourable to the progress of medicine. The nosologist proceeds upon the principle that the characters of disease are, to a certain extent, fixed and

determined, like the botanical characters of a plant, or the chymical properties of a mineral. Hence it too frequently happens that individual cases are compared with the system, instead of the system being corrected by further observation. In this manner young practitioners are in danger of attempting to ascertain a disease by its agreement with the nosological characters, and are drawn away from that minute attention to the phenomena which alone can lead to correct diagnosis. Thus, a medical man might argue with regard to a case indicating disease in the brain, that there can be no effusion, because the pulse has never been below the natural standard, or because the pupils are not dilated; or, with regard to an affection of the abdomen, that there is no inflammation, because the pulse is strong and the bowels open. Nosology, it is true, teaches him that in hydrocephalus, at a certain period, the pulse becomes slow, and the pupils dilated; and that, in intestinal inflammation, the pulse is small and the bowels obstructed; but no great extent of observation is required to show that the symptoms now mentioned are not uniform or essential to these diseases. Such a confidence in system must be equally injurious to the improvement of the individual, and to the progress of medical science; and the examples now given will be sufficient to illustrate the importance of the rule which these observations are intended to convey, separating facts which are occasional or incidental from those which are uniform and essential.

On this subject I shall only add the following anecdote, which I lately received from a medical man of very high intelligence. At an early period of his career as a naval surgeon he was left in charge of a ship on the West India station, when several sailors presented themselves with an affection of the legs, the nature of which was entirely new to him. Having expressed his difficulty to one of the officers, not medical, he was promptly told that the disease was

scurvy, and that if he examined the gums of his patients he would find sufficient evidence. To this he replied that the thing was impossible, because in the nosology of Dr. Cullen it was expressly specified that scurvy occurs "in regione frigida." He was, however, soon convinced that the disease was really scurvy, though it occurred in the West Indies; and, as he added, received a most important lesson,-to observe for himself, instead of trusting to systems.

SECTION III.

OF TRACING AMONG FACTS THE RELATION OF CAUSE AND EFFECT.

OUR knowledge of cause and effect, in reference to any two particular events, is founded entirely upon the observation of a uniform sequence of the events; or of the one following the other in a uniform manner in a great number of instances. The greater the number of instances is in which the sequence has taken place, with the greater confidence, as formerly remarked, do we expect it to take place again under similar circumstances; and every single instance in which it does not occur weakens this confidence, unless we can discover some adequate cause by which the sequence was interrupted. The result of this confidence is, that when we observe the first of two such events, we expect the second to follow it; and that when we observe the second, we conclude the first has preceded it: the first we call cause, the second effect.

In every department of science it is a step of the utmost delicacy to assign to two events this relation; and manifold errors arise from assigning it on inade

quate grounds, that is, on an insufficient number of observations. In medical science we have further to contend with peculiar difficulties and sources of error. These have been already mentioned as referable to two classes,-namely, the difficulty of tracing effects to their true causes, and causes to their true effects; and the manner in which the real tendencies of antecedents or causes are modified or counteracted by a new series of causes which elude our observation. From these peculiarities it often happens that the true antecedents of important events are of an obscure and hidden nature; while the apparent relations would lead us to associate them with antecedents more immediately under our view, but whose connexion with the results is entirely incidental. Other obstacles arise from difficulty in ascertaining the facts themselves, and in tracing the order of the sequences; as, in doing so, we are often obliged to trust to obscure indications of actions which are going on in internal parts, and which are themselves liable to much uncertainty. Thus, a complicated source of difficulty pervades the whole subject of medical causation, and makes it one of the most delicate topics that can engage the attention of the philosophical inquirer.

There are three particular views in which, in medical investigations, we have occasion to trace among successive events the relation of uniform sequence, -namely, the effects of external agents as causes of disease, the effects of external agents as remedies, and the connexion of certain morbid conditions of internal organs with certain symptoms by which these become known to us. In regard to all these objects of research, it is of importance to keep in mind the sources of fallacy to which we are liable, in assigning to a succession of events the relation of uniform sequence, or, in other words, in considering the one as the cause of the other.