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rangement it introduces among our ideas. In this respect even a hypothetical theory may facilitate the recollection of facts; in the same manner in which the memory is aided in remembering the objects of natural history by artificial classifications.

The advantages, however, we derive from true philosophy, are incomparably greater than what are to be expected from any hypothetical theories, These, indeed, may affist us in recollecting the particulars 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 reasoning a priori, An example will illustrate and confirm this observation.

Suppofe 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 mercu. ry 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 refifts the preffure 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 fpaces 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 we obtain an algebraical equation, which affords an easy solution of the problern. It is further evident, that iny knowledge of the physical laws which are here combined, puts it in my power to foretell 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 solved 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 determine, 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 prosecute his discoveries synthetically, in those parts of the universe which he has no access to examine by immediate observation. There is, therefore, this important difference between a 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 experi

There are fome sciences, in which hypothetical theories are more useful than in others : shofe scien. çes, to wit, in which we have occasion for an exten

erce.

live 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 meth. od 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 agricul. ture. In the merely speculative parts of phisics and chemistry, we may go on patiently accuniulating facts, without forming any one conclusion, farther than our facts authorize us ; and leave to posterity the credit of establishing the theory to which our labors are subfervient. 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 wisdom. 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 loosely about him, that either they did not influence his practice at all, or he could eam fily abandon them, whenever they would not bend to his experience.

* Lectures on the Duties and Qualifications of a Physician.

SECTION V.

Continuation of the fame subject.-Effects pr

produced on the Memory by committing to Writing our acquired Knowledge.

HAVING treated at confiderable length of the improvement 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 unfavorable, in some respects, to the faculty of memory, by superfeding, 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 affociation 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 lufficiently illustrated by many authors. Little

attention, 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 founder 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 suggeftions 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 plausible hint that occurs to us! Hardly a day passes, when many such do not occur to ourselves, or are suggeft. ed by others; and detached and insulated, as they may appear at present, some of them may perhaps afterwards, at the distance of years, furnish the keystone of an important system.

But it is not only in this point of view that the philosopher derives advantage from the practice of writing. Without its assistance, he could seldom be able to advance beyond those simple elementary truths which are current in the world, and which form, in the various branches of science, the established creed of the age he lives in. How inconsiderable would have been the progress of mathematicians,

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