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we are already acquainted with ; but it is only from the laws of nature, which have been traced analytically from facts, that we can venture, with safety, to deduce consequences by reasoniug a priori. An ex. ample will illustrate and confirm this observation.
Suppose that a glass tube, thirty inches long, is filled with mercury, excepting eight inches, and is inverted as in the Torricellian experiment, so that the eight inches of common air may rise to the top; and that I wish to know at what height the mercury will remain suspended in the tube, the barometer being at that time twenty-eight inches high. There is here a combination of different laws, which it is necessary to attend to, in order to be able to predict the result. 1. The air is a heavy fluid, and the pressure of the atmosphere is measured by the column of mercury in the barometer, 2. The air is an elastic fluid; and its elasticity at the earth's surface (as it resists the pressure of the atmosphere) is measured by the column of mercury in the barometer. 3. In different states, the elastic force of the air is reciprocally as the spaces which it occupies. But, in this experiment, the mercury which remains suspended in the tube, together with the elastic force of the air in the top of the tube, is a counterbalance to the pressure of the atmosphere ; and therefore their joint effect must be equal to the pressure of a column of mercury twenty-eight inches high. Hence
Hence we obtain an algebraical equation, which affords an easy solution of the problem. It is further evident, that my krowledge of the physical laws which are here combined, puts it in my power to foretel the result, not only in this case, but in all
the cases of a similar 'nature which can be supposed. The problem, in any particular instance, might be folved by making the experiment; but the result would be of no use to me, if the slightest alteration were made on the data.
It is in this manner that philosophy, by putting us in possession of a few general facts, enables us to deter. mine, by reasoning, what will be the result of any supposed combination of them, and thus to comprehend an infinite variety of particulars, which no memory, however vigorous, would have been able to retain. In consequence of the knowledge of such general facts the philosopher is relieved from the necessity of treasuring up in his mind, all those truths which are involved in his principles, and which may be deduced from them by reasoning; and he can often profecute his discoveries synthetically, in those parts of the universe which he has no access to examine by immediate obfervation. There is, therefore, this important difference between the hypothetical theory, and a theory obtained by induction ; that the latter not only enables us to remember the facts we already know, but to ascertain by reasoning, many facts which we have never had an opportunity of examining: whereas, when we reason from a hypothesis a priori, we are almost certain of running into error; and, consequently, whatever may be its use to the memory, it can never be trusted to, in judging of cases which have not previously fallen within our experience.
There are some sciences, in which hypothetical theories are more useful than in others ; those sciences, to wit, in which we have occasion for an extensive
knowledge and a ready recollection of facts, and which, at the same time, are yet in too imperfect a state to allow us to obtain just theories by the method of induction. This is particularly the case in the science of medicine, in which we are under a necessity to apply our knowledge, such as it is, to practice. It is also, in fome degree, the case in agriculture. In the merely speculative parts of physics and chemistry, we may go on patiently accumulating facts, without forming any one conclusion, farther than our facts authorise us; and leave to posterity the credit of establishing the theory to which our labours are subservient. But in medicine, in which it is of consequence to have our knowledge at command, it seems reasonable to think, that hypothetical theories may be used with advantage; provided always, that they are considered merely in the light of artificial memories, and that the student is prepared to lay them aside, or to correct them, in proportion as his knowledge of nature becomes more extensive. I am, indeed, ready to confess, that this is a caution which it is more easy to give than to follow: for it is painful to change any of our habits of arrangement, and to relinquish those systems in which we have been educated, and which have long flattered us with an idea of our own wisdon. Dr. Gregory mentions * it as a striking and distinguishing circumstance in the character of Sydenham, that, although full of hypothetical reasoning, it did not render him the less attentive to observation; and that his hypotheses seem to have fat so
* Lectures on the Duties and Qualifications of a Physician.
loosely loosely about him, that either they did not influence his practice at all, or he could easily abandon them, whenever they would not bend to his experience.
Continuation of the fame Subject.-Effeas produced on the Memory
by committing to Writing our acquired Knowledge.
provement of memory, it may not be improper, before leaving this part of the subject, to consider what effects are likely to be produced on the mind by the practice of committing to writing our acquired knowledge. That such a practice is unfavourable, in some respects, to the faculty of memory, by superseding, to a certain degree, the necessity of its exertions, has been often remarked, and I believe is true; but the advantages with which it is attended in other respects, are so important, as to overbalance greatly this trifling inconvenience.
It is not my intention at present to examine and compare together the different methods which have been proposed, of keeping a common-place book. In this, as in other cases of a similar kind, it may be difficult, perhaps, or impossible, to establish any rules which will apply universally. Individuals must be left to judge for themselves, and to adapt their contrivances to the particular nature of their literary pursuits, and to their own peculiar habits of association and arrangement. The remarks which I am to offer
are very general, and are intended merely to illustrate a few of the advantages which the art of writing affords to the philosopher, for recording, in the course of his progress through life, the results of his speculations, and the fruits of his experience.
The utility of writing, in enabling one generation to transmit its discoveries to another, and in thus giving rise to a gradual progress in the species, has been sufficiently illustrated by many authors. Little atten. tion, however, has been paid to another of its effects, which is no less important; I mean, to the foundation which it lays for a perpetual progress in the intellectual powers of the individual.
It is to experience, and to our own reflections, that we are indebted for by far the most valuable part of our knowledge: and hence it is, that although in youth the imagination may be more vigorous, and the genius more original, than in advanced years ; yet, in the case of a man of observation and inquiry, the judgment may be expected, at least as long as his faculties remain in perfection, to become every day sounder and more enlightened. It is, however, only by the constant practice of writing, that the results of our experience, and the progress of our ideas, can be accurately recorded. If they are trusted merely to the memory, they will gradually vanish from it like a dream, or will come in time to be so blended with the suggestions of imagination, that we shall not be able to reason from them with any degree of confidence. What improvements in science might we not flatter ourselves with the hopes of accomplishing, had we only activity and industry to treasure up every