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application, in any particular instance, does not depend on the contiguity of the two events in place o time, but solely on this question, whether the one event be the constant and invariable forerunner of the other, so that it may be considered as its infallible sign?—Notwithstanding, however, the evidence of this conclusion, philosophers have in general proceeded upon a contrary supposition; and have discovered an unwillingness, even in physics, to call one event the cause of another, if the smallest interval of space or time existed between them. In the case of motion, communicated by impulse, they have no scruple to call the impulse the cause of the motion ; but they will not admit that one body can be the cause of motion in another, placed at a distance from it, unless a connexion is carried on between them, by means of some intervening medium.

It is unnecessary for me, after what has already been said, to employ any arguments to prove, that the communication of motion by impulse, is as unaccountable as any other phenomenon in nature. Those philofophers who have attended at all to the subject, even they who have been the least sceptical with respect to cause and effect, and who have admitted a necessary connexion among physical events, have been forced to acknowledge, that they could not discover any necessary connexion between impulse and motion. Hence, some of them have been led to conclude, that the impulse only rouses the activity of the body, and that the subsequent motion is the effect of this activity, constantly exerted. “ Motion,” says one writer," is action; and a continued motion implies a

" conti

“ continued action.” “ The impulse is only the cause “ of the beginning of the motion : its continuance must “ be the effect of some other cause, which continues “ to act as long as the body continues to move."The attempt which another writer of great learning has made, to revive the ancient theory of mind, has arisen from a similar view of the subject before us. He could discover no necessary connexion between impulse and motion; and concluded, that the impulse was only the occasion of the motion, the beginning and continuance of which he ascribed to the contine 20 nued agency of the mind with which the body is animated.

Although, however, it be obvious, on a moment's consideration, that we are as ignorant of the connexion between impulse and motion, as of the connexion between fire and any of the effects we see it produce, philosophers, in every age, seem to have confidered the production of motion by impulse, as almost the only physical fact which stood in need of no explanation. When we see one body attract another at a distance, our curiosity is roused, and we inquire how the connexion is carried on between them. But when we see a body begin to move in consequence of an impulse which another has given it, we inquire no farther : on the contrary, we think a fact sufficiently accounted for, if it can be shewn to be a case of im. ; pulse. This distinction, between motion produced by impulse, and the other phenomena of nature, we are led, in a great measure, to make, by confounding together efficient and physical causes ; and by applying to the latter, maxims which have properly a re

ference

ference only to the former.—Another circumstance, likewise, has probably considerable influence : that, as it is by means of impulse alone, that we ourselves have a power of moving external objects; this fact is more familiar to us from our infancy than any other ; and strikes us as a fact which is necessary, and which could not have happened otherwise. Some writers have even gone so far as to pretend that, although the experiment had never been made, the communication of motion by impulse, might have been predieted by reasoning a priori *.

From the following pallage, in one of Sir Isaac Newton's letters to Dr. Bentley, it appears, that he supposed the communication of motion by impulse, to be a phenomenon much more explicable, than that a connexion should subsist between two bodies placed at a distance from each other, without any intervening medium. “ It is inconceivable,” says he, “ that in“ animate brute matter should, without the mediation 6 of something else which is not material, operate

upon, and affect other matter, without mutual con“ tact; as it must do, if gravitation, in the sense of

Epicurus, be effential and inherent in it. And this “ is one reason why I desired that you would not " ascribe innate gravity to me. That gravity should “ be innate, inherent, and essential to matter, so that “ one body nay act on another, through a vacuum, “ without the mediation of any thing else, by and “ through which their action and force may be con

* See an Answer to Lord Kaims's Elay on Motion ; by John Stewart, M. D.

“ veyed “ veyed from one to another, is to me so great an

absurdity, that I believe no man who has, in phi

losophical matters, a competent faculty of thinking, can ever fall into it.”

With this passage I so far agree, as to allow that it is impossible to conceive, in what manner one body acts on another at a distance, through a vacuum. But I cannot admit that it removes the difficulty to suppose, that the two bodies are in actual contact. That one body may be the efficient cause of the movie tion of another body placed at a distance from it, I , do by no means assert; but only, that we have as good reason to believe that this may be possible, as to believe that any one natural event is the efficient cause of another.

I have been led into this very long disquisition, concerning efficient and physical causes, in order to point out the origin of the common theories of per. ception; all of which appear to me, to have taken rise from the same prejudice, which I have already remarked to have had so extensive an influence upon the speculations of natural philosophers.

That, in the case of the perception of distant objects, we are naturally inclined to suspect, either something to be omitted from the object to the organ of sense, or some medium to intervene between the object and organ, by means of which the former may communicate an impulse to the latter; appears from the common modes of expression on the subject, which are to be found in all languages. In our own, for example, we frequently hear the vulgar speak, of light striking the eye; not in consequence of any

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philosophical theory they have been taught, but of their own crude and undirected speculations. Per. haps there are few men among those who have attended at all to the history of their own thoughts, who will not recollect the influence of these ideas, at a period of life long prior to the date of their philosophical studies. Nothing, indeed, can be conceived more simple and natural than their origin. When an object is placed in a certain situation with respect to a particular organ of the body, a perception arises in the mind: when the object is removed, the perception ceases.

* Hence we are led to apprehend some connexion between the object and the perception; and as we are accustomed to believe, that matter produces its effects by impulse, we conclude that there must be some material medium intervening between the object and organ, by means of which the impulse is communicated from the one to the other. -That this is really the case, I do not mean to difpute. I think, however, it is evident, that the ex. istence of such a medium does not in

any a priori ; and yet the natural prejudices of men have given rise to an universal belief of it, long before they were able to produce any good arguments in support of their opinion.

case appear

Tum porro varios rerum sentimus odores,
Nec tamen ad nareis venienteis cernimus unquam :
Nec.calidos æltus tuimur, nec frigora quimus
Usurpare oculis, nec voces cernere suemus ;
Quæ tamen omnia corporcâ conftare necesse 'st
Naturâ ; quoniam sensus impellere poffunt.

Lucret. lib. i. p. 299.

Nor

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