Chap. I. application, in any particular inftance, does not depend on the contiguity of the two events in place or time, but folely on this question, whether the one event be the conftant and invariable forerunner of the other, so 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 discovered an unwillingness, even in phyfics, to call one event the cause of another, if the smallest 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 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 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 impulse, 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 least sceptical with respect to cause and effect, and who have admitted a neceffary connexion among phyfical 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 impulfe 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 continued motion implies a " conti " continued action." "The impulfe is only the cause "of the beginning of the motion: its continuance must "be the effect of fome other caufe, 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 arifen from a fimilar view of the subject before us. He could discover no neceffary connexion between impulfe and motion; and concluded, that the impulse 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, seem to have confidered the production of motion by impulfe, as almost the only phyfical fact which stood in need of no explanation. When we fee one body attract another at a distance, our curiofity is roused, and we inquire how the connexion is carried on between them. But when we see 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 distinction, between motion produced by impulfe, 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 re ference Chap. I. ference only to the former.-Another circumftance, likewife, 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 strikes us as a fact which is neceffary, and which could not have happened other wife. Some writers have even gone fo far as to pretend that, although the experiment had never been made, the communication of motion by impulfe, might have been predicted by reasoning a priori *. 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 fhould fubfift between two bodies placed at a distance from each other, without any intervening medium. "It is inconceivable," fays he, " that in"animate brute matter fhould, without the mediation "of fomething else which is not material, operate 66 66 upon, and affect other matter, without mutual con"tact; as it must 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 thing else, by and "through which their action and force may be con * See an Answer to Lord Kaims's Effay on Motion; by John Stewart, M. D. Iveyed 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 distance, 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 cause 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 physical causes, 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 upon the speculations of natural philofophers. That, in the cafe of the perception of diftant objects, we are naturally inclined to fufpect, either fomething 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 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 striking the eye; not in confequence of any philofo 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 studies. Nothing, indeed, can be conceived more fimple and natural than their origin. When an object is placed in a certain fituation with respect to a particular organ of the body, a perception arifes 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 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 to dif pute. I think, however, it is evident, that the ex. istence of such a medium does not in any cafe appear a priori; and yet the natural prejudices of men have given rife to an universal belief of it, long before they were able to produce any good arguments in fupport of their opinion. Tum porro varios rerum fentimus odores, Nec tamen ad nareis venienteis cernimus unquam : LUCRET. lib. i. p. 299. |