substances, and farther observations with the same substances; and it being found that the attraction took place between iron and the magnet only, and that between these it took place in every instance, the general principle would be deduced, or the fact universally true in all instances, that the magnet attracts iron. The same observation applies to the other remarkable property derived from the magnet, namely, pointing to the north. The phenomenon received the name of magnetism, and the laws were then investigated by which it was regulated; but what we call magnetism is still nothing more than a mode of expressing the universal fact, that the magnet attracts iron, and points to the north. On what hidden influence these remarkable phenomena depend we are still as ignorant as the man who first observed them; and, however interesting it would be to know it, the knowledge is not necessary to the investigation of the laws of magnetism.

These may, perhaps, be considered as fair examples of the inductive philosophy, as distinguished from the hypothetical systems of the era which preceded it. According to these, the constant aim of the inquirer was the explanation of phenomena; and in the case before us a theory would have been constructed calculated to account for the attraction by the fluxes and refluxes of some invisible fluid or ether, which would have been described with as much minuteness as if there had been real ground for believing it to exist. Strikingly opposed to all such speculations is the leading principle of the inductive philosophy, that the last object of science is to "ascertain the universality of a fact.”

"The study of nature," says an eminent writer, "is the study of facts, not of causes." In conformity with this truth, the objects of science may perhaps be defined to be, to observe facts; to trace their re lations and sequences; and to ascertain the facts which are universal. It consists in simply tracing

the order which is observed by the phenomena of nature; the efficient causes of these phenomena being considered as beyond the reach of the human faculties, and, consequently, not the legitimate objects of scientific inquiry. It is thus strikingly op posed to the old philosophy, the constant aim of which was the explanation of phenomena, and which has therefore received the name of "the philosophy of causes."

This important distinction between induction and hypothetical speculation, which is now so firmly established in other departments of science, it is to be feared has not been so fully recognised in medical investigations. On the contrary, every one who is acquainted with the history of medical doctrines will probably admit that medicine is still deeply tinged with the philosophy of causes; in other words, that there is a remarkable tendency to refer phenomena to certain obscure principles, which cannot be shown to be facts, and, consequently, cannot be considered as the objects of legitimate inquiry. It is unnecessary in this place to refer more particularly to fictitious and hypothetical principles of this description, which, one after another, have held a prominent place in medical science. If the rules of the inductive philosophy are to be applied to medicine, the immediate effect of them must be to banish all such speculations as contrary to the first rules of sound investigation. They are entirely fictitious principles, framed to correspond with the phenomena instead of being deduced from them. It is also in general beyond the reach of observation either to establish or overturn them; and the only mode of detecting their character is to bring them to the test of the inquiry,-Are they facts? and are the facts universal?

The rules to be observed in deducing general principles appear, therefore, to be the following:1. That the principle assumed be itself a fact.

2. That it be true, without a single exception, of all the individual cases; or, in other words, that the fact be universal.

I. The first of these rules is opposed to a practice lately referred to, which must be admitted to have been very prevalent in medical science, namely, that of referring phenomena to fictitious principles which cannot be shown to be facts. Of the principles of this class, which at various periods have held a prominent place in medical doctrine, some have had their day, and are now forgotten; but it may be doubted whether they were inferior in value to those which have succeeded them. We do not now hear of viscidity of the blood, lentor of the fluids, or rigidity of the solids; of morbific matter in the blood, of hot or cold humours, of obstruction of the animal spirits, and other doctrines, by which various phenomena were explained by the inquirers of former times; but, perhaps, those of more recent date can scarcely be considered as more satisfactory. It may certainly, at least, be a question whether we can concede the character of facts to irregular excitement of the nervous system, hepatic derangement, as that term is very commonly employed, and the numerous modifications under which we meet with the doctrines of determination, irritation, congestion, sympathy, and spasm.

II. The second rule is opposed to the error of hasty generalizing, or of deducing a general statement from a limited number of facts. We can avoid this error only by keeping steadily in view that general principles derive their whole value from being universal facts, or facts that are true without a single exception, in regard to all the individual cases to which the principle is meant to apply. When they are deduced prematurely, that is, from a limited number of facts, or a partial view of their

minute characters, they fail entirely of the purposes which they are meant to serve, and, when trusted, lead us into error. I have formerly alluded to several examples of hasty generalizing in medical science. Some writers have maintained that a certain state of rigidity of the limbs is distinctly characteristic of ramollissement of the brain; and others consider every modification of fever as depending upon inflammation of the gastro-intestinal membrane. This rigidity of the limbs is a frequent occurrence in ramollissement of the brain, and in many cases of fever there is disease of the gastro-intestinal membrane. As a part of the history of the affections, therefore, these are important facts; but they are not true of all the cases of ramollissement and of fever, and, consequently, cannot be admitted as general principles in reference to these affections; for though they are facts, the facts are not universal.

In a science such as medicine, indeed, requiring an accumulation of facts which must often be the result of the labour of ages, partial generalizing may sometimes be admitted merely as a help to the memory; provided we keep constantly in view the imperfect nature of such deductions, and be constantly attentive to correct them by farther observations. But when imperfect results of this kind are received as established principles, they retard our progress in search of truth, or even lead us farther and farther away from it. The confidence is truly remarkable with which such premature deductions in medicines are brought forward, and the facility with which they are often received, without examination, as established principles; much laborious investigation indeed is often devoted to no other purpose than showing them to be fallacious. The zeal for hypothetical systems is considerably gone by; but this tendency to unsound generalizing must be viewed as one of the chief errors which at

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present retard the progress of medical science; and it may, perhaps, be contended, that medicine will never attain a place among the inductive sciences till inquirers agree to act steadily upon the rulethat every medical doctrine shall be a fact, and that the fact shall be universal.

There are two respects in which a fondness for generalizing, in medicine, may be abused, and may lead to errors of a practical nature. The one consists in assuming a fact as general which is not really true of all the individual cases;-various examples of this have been already referred to. The other arises from extending a fact or principle which is true of one class of cases to others with which it is not connected. Thus, a medical man, who decides upon general principles without attending to individual facts, may pronounce a patient to labour under consumption, when he perceives expectoration of a purulent character. Admitting that purulent expectoration may occur in all cases of consumption, the sound observer knows, that it is not confined to this disease, but also occurs in others of a much less dangerous character.

For a legitimate theory, then, it is required that the principle which is assumed be true, and that it be common to all the cases. But there are certain instances, in which a principle ascertained to be true in regard to one set of cases may be extended by conjecture to others, in regard to which its existence is only hypothetical. This may be called legitimate hypothesis, or anticipation of principles; and it differs in this respect from the ficti tious theories already referred to, that it is liable to be either established or overturned by the progress of observation. In this manner, the theory of gravi tation was hypothetically extended to the motions of the heavenly bodies long before the observations of Newton had actually established the truth of the