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 fafety, to deduce confequences by reafoniug a priori. An example will illuftrate and confirm this obfervation.

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 fufpended in the tube, the barometer being at that time twenty-eight inches high. There is here a combination of different laws, which it is neceffary to attend to, in order to be able to predict the refult. 1. The air is a heavy fluid, and the preffure of the atmosphere is measured by the column of mercury in the barometer. 2. The air is an elaftic fluid; and its elasticity at the earth's furface (as it refifts the preffure of the atmosphere) is measured by the column of mercury in the barometer. 3. In different ftates, the elastic force of the air is reciprocally as the spaces which it occupies. But, in this experiment, the mercury which remains fufpended in the tube, together with the elastic force of the air in the top of the tube, is a counterbalance to the preffure of the atmosphere; and therefore their joint effect must be equal to the preffure of a column of mercury twenty-eight inches high. Hence we obtain an algebraical equation which affords an eafy folution of the problem. It is further evident, that my knowledge of the phyfical laws which are here combined, puts it in my power to foretel the result, not only in this case, but in all

the

the cafes of a fimilar nature which can be fuppofed. The problem, in any particular inftance, might be folved by making the experiment; but the refult would be of no use to me, if the flightest alteration were made on the data.

It is in this manner that philofophy, by putting us in poffeffion of a few general facts, enables us to determine, by reasoning, what will be the refult of any fupposed 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 fuch general facts the philofopher is relieved from the neceffity of treasuring up in his mind, all those truths which are involved in his principles, and which may be deduced from them by reafoning; and he can often profecute his discoveries fynthetically, 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 reafoning, many facts which we have never had an opportunity of examining: whereas, when we reafon from a hypothefis a priori, we are almost certain of running into error; and, confequently, whatever may be its use to the memory, it can never be trufted to, in judging of cafes which have not previously fallen within our experience.

There are fome fciences, in which hypothetical theories are more useful than in others; those sciences, to wit, in which we have occafion for an extensive

know

knowledge and a ready recollection of facts, and which, at the fame time, are yet in too imperfect a state to allow us to obtain just theories by the method of induction. This is particularly the cafe in the science of medicine, in which we are under a neceflity to apply our knowledge, fuch as it is, to practice. It is alfo, in fome degree, the cafe in agriculture. In the merely fpeculative parts of phyfics and chemistry, we may go on patiently accumulating facts, without forming any one conclufion, farther than our facts authorife us; and leave to pofterity the credit of eftablishing the theory to which our labours are fubfervient. But in medicine, in which it is of confequence to have our knowledge at command, it feems reafonable to think, that hypothetical theories may be used with advantage; provided always, that they are confidered merely in the light of artificial memories, and that the student is prepared to lay them afide, or to correct them, in proportion as his knowledge of nature becomes more extenfive. I am, indeed, ready to confefs, that this is a caution which it is more eafy 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 wifdom. 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 lefs attentive to obfervation; and that his hypothefes feem to have fat fo

Lectures on the Duties and Qualifications of a Physician.

loofely

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.

SECTION V.

Continuation of the fame Subject.-Effects produced on the Memory by committing to Writing our acquired Knowledge.

HAVING

AVING treated at confiderable length of the improvement of memory, it may not be improper, before leaving this part of the subject, to confider what effects are likely to be produced on the mind by the practice of committing to writing our acquired knowledge. That fuch a practice is unfavourable, in fome refpects, to the faculty of memory, by fuperfeding, to a certain degree, the neceffity 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 prefent to examine and compare together the different methods which have been propofed, of keeping a common-place book. In this, as in other cafes of a fimilar kind, it may be difficult, perhaps, or impoffible, to establish any rules which will apply univerfally. Individuals must be left to judge for themselves, and to adapt their contrivances to the particular nature of their literary purfuits, and to their own peculiar habits of affociation and arrangement. The remarks which I am to offer

are

are very general, and are intended merely to illuftrate a few of the advantages which the art of writing affords. to the philofopher, for recording, in the courfe of his progrefs through life, the refults of his fpeculations, and the fruits of his experience.

The utility of writing, in enabling one generation to transmit its discoveries to another, and in thus giving rife to a gradual progrefs in the fpecies, has been fufficiently illuftrated by many authors. Little attention, however, has been paid to another of its effects, which is no lefs important; I mean, to the foundation which it lays for a perpetual progrefs 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 cafe of a man of obfervation 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 progrefs of our ideas, can be accurately recorded. If they are trufted merely to the memory, they will gradually vanish from it like a dream, or will come in time to be fo blended with the fuggeftions of imagination, that we fhall not be able to reafon from them with any degree of confidence. What improvements in fcience might we not flatter ourselves with the hopes of accomplishing, had we only activity and industry to treasure up every plaufible