for stricture or calculus,-and affections of the bowels for diseases of the liver. Many similar examples will occur to those who are conversant with the history of medicine. The error may occur to the respectable practitioner from misapprehension, arising out of the uncertainty of the art; but it also appears to be one of the great resources of the empiric. When we hear, therefore, of marvellous cures of formidable diseases, our first inquiry ought to be, not merely whether the patient recovered, but what evidence there is that the alleged disease ever existed..

Such are the difficulties and uncertainties of medical causation; and such is the ground for caution in considering two events as connected in the manner of cause and effect. Among the sources of this difficulty, there are several circumstances which are entirely beyond our reach, and the influence of which upon our researches we cannot hope entirely to overcome; but, by keeping steadily in view the sources of error by which we are surrounded, we may avoid any very fallacious conclusions, and may make some progress towards the truth. In regard to the effects of medicines, in particular, there are two opposite errors to be equally avoided; namely, an implicit confidence in the power of particular remedies, and a total skepticism in regard to the resources of medicine. Both these extremes are equally unworthy of persons of calm philosophical observation; and they who advance carefully in the middle course, not misled by the temptation to hasty conclusions, and cautioned but not discouraged by the dangers of concluding falsely, are most likely to contribute something towards diminishing the uncertainty of medicine.

Before leaving the subject of causation, I would briefly allude to a confusion which has been introduced into the language of medicine by the division

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of the causes of disease into predisposing, exciting, and proximate. It is well known that a cause which appears to produce a disease in one instance will, in another, be followed by no result, or by a disease of a different kind. Attempts have accordingly been made to investigate the circumstances which produce a tendency to be affected by particular diseases at certain times, and these have received the name of predisposing causes, or sometimes of occasional causes. The effluvia of marshes, for example, are considered as the exciting cause of intermittent fever; but the disease is not produced in all who are exposed to this effluvia. Various circumstances, such as fatigue and intemperance, are said to act as the predisposing or occasional causes. But, in other situations, fatigue and intemperance were never known to produce intermittent fever; and they cannot, therefore, in correct language, be said to be connected with the disease in the manner of cause. The term proximate cause, again, has been applied to minute changes which take place in certain functions of the body so as to constitute particular diseases. Such speculations are, in general, in a great measure hypothetical; but, even if they were ascertained to be true, they must be considered as constituting the nature and essence of the disease, and could not be regarded in the light of a cause. If these observations shall be considered as entitled to any weight, it will follow that the term cause ought to be restricted to that which has commonly been called the exciting cause.

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SECTION IV.

OF DEDUCING GENERAL FACTS OR GENERAL PRINCIPLES.

HAVING, with the cautions which have been referred to, collected a body of authentic facts, and having traced among these facts the relation of uniform sequence; or uniform conjunction, the next step in our investigation is to bring together a number of these facts, or classes of facts, and to endeavour to deduce from them general principles.

By the process of mind which we call abstraction, we separate one property of a substance from its other properties, or one fact from a chain of facts, and contemplate it apart. When we thus view a number of substances, or a number of classes of facts, and separate an individual property or individual fact which is common to them all, we may then contemplate this fact or property as characteristic of the whole class: and the process constitutes generalizing, or deducing a general fact, or general principle.

Generalizing is to be distinguished from classification, though the mental process concerned is in both essentially the same. We class together a certain number of substances by a property in which they agree; and, in doing so, we specify and enumerate the individual substances included in the class. Thus, we may take a number of substances differing widely in their external and mechanical properties, some being solid, some fluid, and some gaseous, and say they are all acids. The class being thus formed, and consisting of a defined number of substances which agree in the property of acidity, we may next investigate some other property which is common

to all the individuals of the class, and belongs to no other, and say, for example, that all acids redden vegetable-blues. The former of these operations is properly classification; the latter is generalizing in reference to the class. In the former, we take or exclude individual substances, according as they possess or not the property on which the classification rests; in performing the latter, the property which is assumed must belong to all the individuals without a single exception, or, if it does not, it must be abandoned as a general fact or general principle in reference to the class. In classifying, we may use every freedom regarding individuals in taking or excluding them. In generalizing, we must not exclude a single individual; for the principle which does not include every one of them, that is, the proposed fact which is not true of all the individuals is not a general fact, and consequently cannot be admitted as a general principle. For in physical science, to talk of exceptions to a general rule is only to say, in other words, that the rule is not general, and, consequently, is unworthy of confidence If one acid were discovered which does not redden vegetable-blues, it would belong to a history of these substances to state that a certain number of them have this property; but the property of reddening vegetable-blues would require to be abandoned as a general fact or general principle applicable to the class of acids.

A general law, or general principle, then, is nothing more than a general fact, or a fact which is invariably true of all the individual cases to which it professes to apply. Deducing such facts is the great object of modern science; and it is by this peculiar character that it is distinguished from the ancient science of the schools, the constant aim of which was to discover causes. The general law of ravitation, for example, is nothing more than the neral fact, or fact invariably true, that all bodies

when left unsupported, fall to the ground. There were at one time certain apparent exceptions to the universality of this law, namely, in some very light bodies, which were not observed to fall. But a little farther observation showed that these are prevented from falling by being lighter than the atmosphere, and that in vacuo they observe the same law as the heaviest bodies. The apparent exceptions being thus brought under the law, it became general, namely, the fact universally true, that all unsupported bodies fall to the ground. Now, of the cause of this phenomenon we know nothing; and what we call the general law, or general principle of gravitation, is nothing more than a universal fact, or a fact that is true without a single exception. But having ascertained the fact to be invariably and universally true, we assume it as a part of the established order of nature, and proceed upon it with as much confidence as if we knew the mysterious agency on which the phenomenon depends. The establishment of the fact as universal brings us to that point in the inquiry which is the limit of our powers and capacities, and it is sufficient to the purposes of science. On the same principle, it is familiar to every one that extensive discoveries have been made in regard to the properties and laws of heat; but we do not know what heat is, whether a distinct essence, or, as has been supposed by some philosophers, a peculiar motion of the minute atoms of bodies.

In the same manner, the person who first observed iron attracted by the magnet observed a fact which was to him new and unaccountable. But the same phenomenon having been observed a certain number of times, a belief would arise that there existed between it and the substances concerned a connexion of cause and effect. The result of this belief would be, that when the substances were brought together the attraction would be expected to take place. Observations would then probably be made with other