which general laws must be refolved; they have a tendency, in many cafes, to counteract the influence of habit, in weakening those emotions of wonder and of curiofity, which the appearances of nature are fo admirably fitted to excite. For this purpofe, it is neceflary, either to lead the attention to facts which are calculated to ftrike by their novelty. or to prefent familiar appearances in a new light; and fuch are the obvious effects of philofophical inquiries; fometimes extending our views to objects which are removed from vulgar obfervation; and fometimes correcting our first apprehenfions with refpect to ordinary events. The communication of motion by impulse, (as I already hinted,) is as unaccountable as any phenomenon we know; and yet, most men are difpofed to confider it, as a fact which does not refult from will, but from neceffity. To fuch men, it may be useful to direct their attention to the univerfal law of gravitation; which, although not more wonderful in itself, than the common effects of impulfe, is more fitted, by its novelty, to awaken their attention, and to excite their curiofity. If the theory of Bofcovich fhould ever be established on a fatisfactory foundation, it would have this tendency in a ftill more remarkable degree, by teaching us that the communication of motion by impulfe, (which we are apt to confider as a neceffary truth,) has no existence whatever; and that every cafe in which it appears to our senses to take place, is a phenomenon no less inexplicable, than that principle of attraction which binds together the moft remote parts of the universe.

If fuch, however, be the effects of our philofophical pursuits when fuccefsfully conducted, it must be confeffed that the tendency of imperfect or erroneous theories is widely different. By a fpecious folution of infuperable difficulties, they fo dazzle and bewilder the understanding, as, at once, to prevent

us from advancing, with steadiness, towards the limit of human knowledge; and from perceiving the existence of a region beyond it, into which philofophy is not permitted to enter. In fuch cafes, it is the bufinefs of genuine fcience to unmask the impofture, and to point out clearly, both to the learned and to the vulgar, what reafon can, and what the cannot, accomplish. This, I apprehend, has been done, with refpect to the hiftory of our perceptions, in the most fatisfactory manner, by Dr. Reid. When a perfon little accustomed to metaphyfical fpeculations is told, that, in the cafe of volition, there are certain invifible fluids, propagated from the mind to the organ which is moved; and that, in the cafe of perception, the existence and qualities of the external object are made known to us by means of fpecies, or phantafms, or images, which are present to the mind in the fenforium; he is apt to conclude, that the intercourfe between mind and matter is much lefs myfterious than he had fuppofed; and that, although thefe expreffions may not convey to him any very diftinct meaning, their import is perfectly understood by philofophers. It is now, I think, pretty generally acknowledged by physiologists, that the influence of the will over the body, is a mystery which has never yet been unfolded; but, fingular as it may appear, Dr. Reid was the first person who had courage to lay completely afide all the common hypothetical language concerning perception, and to exhibit the difficulty in all its magnitude, by a plain ftatement of the fact. To what then, it may be afked, does this ftatement amount ?-Merely to this; that the mind is so formed, that certain impreffions produced on our organs of fenfe by external objects, are followed by correfpondent fenfations; and that these fenfations, (which have no more resemblance to the qualities of matter, than the words of a language have to the

things they denote,) are followed by a perception of the existence and qualities of the bodies by which the impreffions are made; that all the steps of this procefs are equally incomprehenfible; and that, for any thing we can prove to the contrary, the connexion between the fenfation and the perception, as well as that between the impreffion and the fenfation, may be both arbitrary: that it is therefore by no means impoffible, that our fenfations may be merely the occafions on which the correfpondent perceptions are excited; and that at any rate, the confideration of these sensations, which are attributes of mind, can throw no light on the manner in which we acquire our knowledge of the existence and qualities of body. From this view of the subject, it follows, that it is external objects themselves, and not any species or images of these objects, that the mind perceives; and that although, by the conftitution of our nature, certain fenfations are rendered the conftant antecedents of our perceptions, it is juft as difficult to explain how our perceptions are obtained by their means, as it would be, upon the fuppofition, that the mind were all at once inspired with them, without any concomitant fenfations whatever.

These remarks are general, and apply to all our various perceptions; and they evidently ftrike at the root of all the common theories upon the subject. The laws, however, which regulate thefe perceptions, are different in the cafe of the different fenfes, and form a very curious object of philofophical inquiry.-Thofe, in particular, which regulate the acquired perceptions of fight, lead to fome very interesting and important fpeculations; and, I think, have never yet been explained in a manner completely fatisfoctory. To'treat of them in detail, does not fall under the plan of this work; but I fhall have occafion to make a few remarks on them, in the chapter on Conception.

In oppofition to what I have here obferved on the importance of Dr. Reid's fpeculations concerning our perceptive powers, I am fenfible it may be urged, that they amount merely to a negative discovery; and it is poffible, that fome may even be forward to remark, that it was unneceffary to employ fo much labor and ingenuity as he has done, to overthrow an hypothesis of which a plain account would have been a fufficient refutation.-To fuch perfons, I would beg leave to fuggeft, that, although, in confequence of the jufter views in pneumatology, which now begin to prevail, (chiefly, I believe, in confequence of Dr. Reid's writings,) the ideal fyftem may appear to many readers unphilofophical and puerile; yet the cafe was very different when this author entered upon his inquiries: and I may even venture to add, that few pofitive discoveries, in the whole history of science, can be mentioned, which found a jufter claim to literary reputation, than to have detected, fo clearly and unanfwerably, the fallacy of an hypothefis, which has defcended to us from the earlieft ages of philofophy: and which, in modern times, has not only ferved to Berkeley and Hume as the bafis of their fceptical fyftems, but was adopted as an indisputable truth by Locke, by Clarke, and by Newton.

SECTION IV.

Of the Origin of our Knowledge.

THE philofophers who endeavored to explain the operations of the human mind by the theory of ideas, and who took for granted, that in every exertion of thought there exifts in the mind fome object distinct from the thinking fubftance were naturally led to inquire whence thefe ideas derive their origin; in

*particular, whether they are conveyed to the mind from without by means of the fenfes, or from part of its original furniture?

With refpect to this question, the opinions of the ancients were various; but as the influence of thefe opinions on the prevailing fystems of the prefent age is not very confiderable, it is not neceflary, for any of the purposes I have in view in this work, to confider them particularly. The moderns, too, have been much divided on the fubject; fome holding with Des Cartes, that the mind is furnished with certain innate ideas; others, with Mr. Locke, that all our ideas may be traced from fenfation and reflection; and many, (efpecially among the later metaphyficians in France,) that they may be all traced from fenfation alone.

Of thefe theories, that of Mr. Locke deferves more particularly our attention; as it has ferved as the bafis of moft of the metaphysical fyftems which have appeared fince his time; and as the difference between it and the theory which derives all our ideas from fenfation alone, is rather apparent than real.

In order to convey a juft notion of Mr. Locke's doctrine concerning the origin of our ideas, it is neceffary to remark, that he refers to sensation, all the ideas which we are supposed to receive by the external fenfes; our ideas, for example, of colours, of founds, of hardness, of extenfion, of motion; and, in fhort, of all the qualities and modes of matter; to reflection, the ideas of our own mental operations which we derive from consciousness; our ideas, for example, of memory, of imagination, of volition, of pleasure, and of pain. Thefe two fources, according to him, furnish us with all our fimple ideas, and the only power which the mind poffeffes over them, is to perform certain operations, in the way of composition, abftraction, generalisation, &c. on the materials which it thus collects in the course of its experi

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