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tinct impression of the probable result, some self-consistent anticipation, as the ground of the prudens questio (the wellconsidered interrogation), the fore-thoughtful query, which he affirms to be itself the first half of the knowledge or explanation sought. With him an idea in physics is an experiment proposed; an experiment successful, an idea realized, a question answered in the affirmative."1
Thus we see that hypothesis or ingenious conjecture, by which the various possible answers to be given to our questioning of nature are anticipated, is an essential part of the system of induction propounded by Bacon. We are to approach nature with due respect, not rudely asking of her unmeaning questions, else we shall have the fool's answer, and hear nothing but the echo of our last-uttered syllable. We are to meditate well how to frame our speech before we enter into the temple of the oracle; we may then expect to get no ambiguous reply, but just such an answer as covers our question.
The forms or radical causes, primary facts, or laws— these terms being used interchangeably in physics—are of various depth and extension. There are laws or general
facts which Bacon would call collective instances. These may be looked upon as invaluable generalizations, out of which practical directions are derived, and from which we step to the ultimate form or universal law. Some of the most remarkable examples of such general facts or intermediate laws of nature, are exhibited in Kepler's three great discoveries:-1st. That the planets move in elliptical orbits, having the sun for their common focus. 2nd. That the planets describe equal areas in equal times. And 3rd. That the squares of the periodic times of the planets, are as the cubes of their mean distances from the sun. To arrive at these facts, Kepler had to employ the methods first of exclusion, and next of ingenious hypothesis, which, with
1 The Friend. Vol. III., p. 167.
enormous labour, he verified by calculation. The facts themselves were most important, and practically useful; and out of them Newton arrived at his general law of gravitation. To this order would belong any law that indicated the general relationship between morbid actions and curative agencies, as for example, the law proposed to account for the action of specifics, and known as similia similibus curantur.
For to medicine is denied the most potent of all the weapons of the inductive philosophy, the Instantia Crucis, which comes into play when, in the course of our investigation into the causes of a phenomenon, we can reduce the possible explanations to two. All that then remains to be done, is to look out for some fact included in the phenomena, which is explained by one and not by the other of the supposed causes. This choice between the two is like that which a traveller has to make when he comes to the intersection of one road with another; and as he is relieved from his dilemma by a cross indicating the direction of both roads, so is the investigator of science enlightened by this kind of experiment; and hence it has received the name of the experiment, or instance, of the cross, or the crucial experiment. For example, many of the motions of the planets are equally well accounted for by the system of Ptolemy, in which the earth is assumed to be the centre, and by that of Copernicus, where the sun occupies that position; but there is one particular group of phenomena, known to astronomers as the digression of the planets from the plane of the ecliptic, which cannot be reconciled with the Ptolemaic theory, but finds an instant explanation if we adopt that of Copernicus. Such an observation would be called by Bacon, Instantia Crucis. The experimentum crucis consists in making two experiments exactly like one another in every particular but one. This, however, is impossible, unless we can
command all the conditions of both experiments, which cannot be done in medicine. Suppose the question we want to resolve is the efficacy of some particular remedy in the cure of a given disease. The two facts are the course of the disease when the remedy is administered to the patient, and the course of the disease without the remedy; but the other causes which combine in producing a recovery or the reverse, and in modifying the progress and event of every particular case, are so numerous and uncontrollable, that it is impossible to institute a crucial experiment—that is, two experiments, corresponding in every condition, except the one in question.'
Although the inductive method, as proposed by Bacon, has not led directly to the greatest discoveries in therapeutics, it would be unjust to his memory not to give him credit for a large share in the general advancement of the art of medicine, by the clear and emphatic way in which he pointed out its defects, and laid down the rules, by attending to which we may attain the greatest amount of security and certainty. As one of the suggestions of Bacon, in regard to the form or principle of colour, was the germ out of which Newton's great discovery of the composition of light arose, so we may venture to affirm that the greatest improvements in medicine have been made in accordance with the directions given by Bacon for the successful prosecution of the study of the art of healing.
Thus, in regard to anatomy, he observes:-" As for the passages and pores, it is true, which was anciently noted, that the more subtle of them appear not in anatomies, because they are shut up and latent in dead bodies, though they may be open and manifest in life." The passages which are open in living, and closed in dead bodies, are the arteries. To them Bacon directed the special attention of
1 Playfair's Dissertation, Mill's Logic, 2 Advancement of Learning, p. 171.
the investigator; and while he agreed with Celsus in disapproving of the dissection of living men, yet he could see no reason why the experiments required to determine the use of these vessels should not be made on the lower animals. It was by pursuing the investigation of these very arteries, according to the letter of instruction given by Bacon, that Harvey made his discovery of the circulation of the blood.
In regard to pathology, he writes:-"As for the footsteps of disease, and their devastations of the inward parts, they ought to have been exactly observed by multitudes of anatomies, and the contributions of men's several experiences, and carefully set down, both historically, according to the appearances, and artificially, with a reference to the diseases and symptoms which result from them, in case when the anatomy is of a defunct patient; whereas, now they are passed over slightly and in silence."1
Here we have an exact description of what modern morbid anatomy and pathology are occupied with; but two hundred years elapsed between the injunction by Bacon and its successful fulfilment. Of medicine, in general, he observes:-"Of all substances which nature hath produced, man's body is the most extremely compounded ; for we see herbs and plants are nourished by earth and water; beasts for the most part, by herbs, and fruits ; man by the flesh of beasts, birds, fishes, herbs, grains, fruits, water, and the manifold alterations, dressings, and preparations of the several bodies, before they come to be his food and aliment. Add hereunto, that beasts have a more simple order of life, and less change of affections to work upon their bodies; whereas man, in his mansion, sleep, exercise, passions, hath infinite variation. . This variable composition of man's body has made it an instrument easy to distemper, and therefore the poets do well to conjoin
1 Advancement of Learning, p. 173.
music and medicine in Apollo, because the office of medicine is but to tune this curious harp of man's body, and to reduce it to harmony. So that the subject, being so variable, hath made the art by a consequence more conjectural, and the art, being conjectural, hath made so much the more place to be left for imposture. For almost all other arts and sciences are judged by acts or masterpieces, as I may term them, and not successes and events. The lawyer is judged by the virtue of his pleadings, and not by the issue of the cause. But the physician hath no particular arts demonstrative of his ability, but is judged most by the event. This is the reason why the physician, seeing that it befalleth to him, even as to the fool, in his own profession, and modest merit outstripped by impudent presumpsion, is apt to give himself up to other pursuits besides those of a purely professional character. Though natural, this is not commendable; for nothing can be more variable than faces, and yet memory can retain them and distinguish them; nothing more variable than voices, yet men can discern them; nothing more variable than the sound of words, yet they have been reduced to a few simple letters; so that it is not owing to the incapacity of the mind of man, but because he has not closely observed the varieties of diseases and adapted his remedies accordingly; as the poet says::
"Et quoniam variant morbi variabimus artes
Mille mali species mille salutis erunt.'" 1
To examine minutely the various forms of disease, and to adapt to each its own particular remedy, is the general instruction given by Bacon for advancing medicine.
The following passage, although uttered with rhetorical emphasis, probably expresses Bacon's deliberate estimate of Galen and his system:-"This is the man that would screen the ignorance and sloth of physicians from their 1 Op. cit.