ble forerunner of the other, fo that it may be confidered as its infallible fign?-Notwithstanding, however, the evidence of this conclufion, philofophers have in general proceeded upon a contrary fuppofition; and have difcovered an unwillingness, even in phyfics, to call one event the cause of another, if the fmalleft interval of space or time exifted between them. In the cafe of motion, communicated by impulfe, they have no fcruple to call the impulfe the caufe 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 fome intervening medium. It is unneceffary for me, after what has already been faid, to employ any arguments to prove, that the communication of motion by impulfe, is as unaccountable, as any other phenomenon in nature. Those philofophers who have attended at all to the fubject, even they who have been the leaft fceptical with refpect to caufe and effect, and who have admitted a neceffary connection among physical events, have been forced to acknowledge, that they could not discover any neceffary connexion between impulfe and motion. Hence, fome of them have been led to conclude, that the impulse only roufes the activity of the body, and that the fubfequent motion is the effect of this activity, conftantly exerted. Motion," fays one writer, "is action; and a con"tinued motion implies a continued action." "The "impulfe is only the caufe of the beginning of the "motion; its continuance must be the effect of fome "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 fimilar view of the fubject before us. He could discover no neceffary connection between impulfe and -K motion; and concluded, that the impulfe was only the occafion of the motion, the beginning and continuance of which he afcribed to the continued agency of the mind with which the body is animated. Although, however, it be obvious, on a moment's confideration, that we are as ignorant of the connexion between impulfe and motion, as of the connexion between fire and any of the effects we fee it produce, philofophers, in every age, feem to have confidered the production of motion by impulfe, as almoft the only phyfical fact which stood in need of no explanation. When we fee one body attract another at a diftance, our curiofity is roufed, and we inquire how the connexion is carried on between them. But when we fee a body begin to move in confequence of an impulse which another has given it, we inquire no farther: on the contrary, we think a fact fufficiently accounted for, if it can be fhewn to be a case of impulfe. This diftinction, 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 phyfical caufes; and by applying to the latter, maxims which have properly a reference only to the former. -Another circumftance, likewise, has probably confiderable influence: that, as it is by means of impulfe 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 ftrikes us as a fact which is neceffary, and which could not have happened otherwife. Some writers have even gone fo far as to pretend that, although the experiment had never been made, the communication of the motion by impulfe, might have been predicted by reafoning a priori.* *See an Answer to Lord Kaims's Essay on motion; by John Stewart, M. D. From the following paffage, in one of Sir Ifaac Newton's letters to Dr. Bentley, it appears that he fuppofed the communication of motion by impulfe, to be a phenomenon much more explicable, than that a connexion should fubfift between two bodies placed at a distance from each other, without any intervening medium. "It is inconceivable," fays he, "that inanimate brute matter fhould, without the "mediation of something else which is not material, operate upon, and affect other matter, without "mutual contact; as it muft do, if gravitation, in "the fense of Epicurus, be effential and inherent in "it. And this is one reason why I defired that you "would not afcribe innate gravity to me. That "gravity fhould be innate, inherent, and effential to "matter, fo that one body may act on another, "through a vacuum, without the mediation of any 66 thing elfe, by and through which their action and "force may be conveyed from one to another, is to "me fo great an abfurdity, that I believe no man "who has, in philofophical matters, a competent "faculty of thinking, can ever fall into it." With this paffage I fo far agree, as to allow that it is impoffible to conceive, in what manner one body acts on another at a diftance, through a vacuum. But I cannot admit that it removes the difficulty to fuppofe that the two bodies are in actual contact. That one body may be the efficient caufe of the motion of another body placed at a distance from it, I do by no means affert; but only, that we have as good reason to believe that this may be poffible, as to believe that any one natural event is the efficient cause of another. I have been led into this very long difquifition, concerning efficient and phyfical caufes, in order to point out the origin of the common theories of perception; all of which appear to me to have taken rife from the fame prejudice, which I have already remarked to have had fo extenfive an influence up. on the fpeculations of natural philofophers. That, in the cafe of the perception of diftant ob jects, we are naturally inclined to fufpect, either fomething to be emitted from the object to the organ of fenfe, or fome medium to intervene between the object and organ, by means of which the former may communicate an impulfe to the latter; appears from the common modes of expreffion on the fubject, which are to be found in all languages. In our own, for example, we frequently hear the vulgar fpeak, of light ftriking the eye; not in confequence of any philofophical theory they have been taught, but of their own crude and undirected fpeculations. Perhaps there are few men among those who have attended at all to the hiftory 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 philofophical ftudies. Nothing, indeed, can be conceiv. ed more fimple and natural than their origin. When an object is placed in a certain fituation with refpect to a particular organ of the body, a perception arises in the mind; when the object is removed, the ception ceases. *Hence we are led to apprehend fome connexion between the object and the perception; and as we are accustomed to believe, that matter produces its effects by impulfe, we conclude that there must be fome 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 cafe, I do not mean *Tum porrò varios rerum sentimus odores, A Nec tamen ad nareis venienteis cernimus unquam : Nec calidos æstus tuimur, nec frigora quimus LUCRET. lib. i. P. 299. per to difpute. I think, however, it is evident, that the existence of fuch a medium does not in any cafe appear a priori; and yet the natural prejudices of men have given rife to an univerfal belief of it, long before they were able to produce any good arguments in fupport of their opinion. Nor is it only to account for the connexion, between the object and the organ of fenfe, that philofophers have had recourse to the theory of impulfe. They have imagined that the impreffion on the organ of fenfe is communicated to the mind, in a fimilar manner. As one body produces a change in the ftate of another by impulfe, fo it has been fuppofed, that the external object produces perception, (which is a change in the ftate of the mind,) first, by fome material impreffion made on the organ of fenfe; and, fecondly, by fome material impreffion communicated from the organ to the mind along the nerves and brain. These fuppofitions, indeed, as I had occafion already to hint, were, in the ancient theories of perception, rather implied than expreffed; but by modern philofophers, they have been stated in the form of explicit propofitions. "As to the man"ner," fays Mr. Locke," in which bodies produce "ideas in us; it is manifeftly by impulfe, the only way which we can conceive bodies operate in."* And Sir Ifaac Newton, although he does not speak of an impulfe made on the mind, plainly proceeded on the principle that, as matter can only move matter by impulfe, fo no connexion could be carried on between matter and mind, unless the mind were prefent (as he expreffes it) to the matter from which the laft impreffion is communicated. "Is not" (fays he)" the fenforium of animals, the place where the "fentient fubftance is prefent; and to which the "fenfible fpecies of things are brought, through the * Essay on Human Understanding, book ii. chap. viii. § 11. * |