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ble forerunner of the other, so that it may be confid. ered as its infallible fign?-Notwithstanding, however, the evidence of this conclusion, philosophers have in general proceeded. upon a contrary fuppofition; 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 fcruple 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 impulfe, is as unaccountable, as any other phenomenon in nature. Those philosophers 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 ad. mitted a necessary connection 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 con“ tinued motion implies a 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 fimi"lar view of the subject before us. He could discov. er no necessary connection between impulfe and
motion ; and concluded, that the impulse was only the occasion of the motion, the beginning and continuance of which he ascribed to the continued 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 considered the production of inotion by impulse, as almost the only phyfical 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 fee 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 impulse. 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 physical causes; and by applying to the latter, maxims which have properly a reference only to the former.
-Another circumstance, likewise, has probably conliderable infuence : 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 fo far as to pretend that, although the experiment had never been made, the communication of the motion by impulse, might have been predicted by reasoning a priori.*
* See an Answer to Lord Kaims's Esay on motion ; by John Stewart, M. D.
From the following passage, 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 sublist between two bodies placed at a distance from each other, without any intervening medium. “ It is inconceivable,” says he, " that inanimate brute matter should, without the “ mediation of something else which is not material,
operate upon, and affect other matter, without “ mutual contact; as it must do, if gravitation, in “the sense of Epicurus, be essential and inherent in - it. And this is one reason why I desired that you 66 would not ascribe innate gravity to me.
That gravity should be innate, inherent, and effential to
matter, fo that one body may act on another, " through a vacuuin, without the mediation of any “ thing else, by and through which their action and “ force may be conveyed from one to another, is to “me so great an absurdity, that I believe no man " who has, in philosophical matters, a competent s faculty of thinking, can ever fall into it.”
With this passage I so 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 suppose 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 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 perception; all of which appear to me to have taken rise from the same prejudice, which I have already
remarked to have had fo extensive an influence upon the speculations of natural philosophers.
That, in the case of the perception of diftant objects, we are naturally inclined to suspect, either fomething to be emitted from the object to the or. gan of lense, 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 philosophical theory they have been taught, but of their own crude and undirected speculations. Perhaps 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 philofophical studies. Nothing, indeed, can be conceiv. ed 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 per. ception 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 dispute. I think, however, it is evident, that the existence of such a medium does not in any case appear a priori ; and yet the natural prejudices of men have given rise to an univerfal belief of it, long before they were able to produce any good arguments in support of their opinion.
* Tum porrò varios rerum sentimus odores,
Nec tamen ad nareis venienteis cernimus unquam :
LUCRET, lib. i. p. 299.'
Nor is it only to account for the connexion, between the object and the organ of sense, that philofophers have had recourse to the theory of impulse. They have imagined that the impression on the organ of sense is communicated to the mind, in a fimilar manner.
As one body produces a change in the state of another by impulse, so it has been supposed, that the external object produces perception, (which is a change in the state of the mind,) first, by some material impression made on the organ of sense; and, secondly, by some material impression communicated from the organ to the mind along the nerves and brain. These fuppositions, indeed, as I had occafion already to hint, were, in the ancient theories of perception, rather implied than expressed; but by modern philosophers, they have been stated in the form of explicit propositions. “ As to the man“ ner,” says Mr. Locke, “ in which bodies produce “ ideas in us ; it is manifestly by impulse, the only “ way which we can conceive bodies operate in." And Sir Isaac Newton, although he does not speak of an impulse made on the mind, plainly proceeded on the principle that, as matter can only move matter by impulse, so no connexion could be carried on between matter and mind, unless the mind were present (as he expresses it) to the matter from which the last impression is communicated. “Is not" (fays he) “ the fenforium of animals, the place where the .“ lentient substance is present; and to which the “ sensible fpecies of things are brought, through the
* Essay on Human Understanding, book ii. chap. viii. $11.