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 so admirably fitted to excite. For this purpose, it is neceffary, either to lead the attention to facts which are calculated to strike by their novelty. or to present familiar appearances in a new light; and such are the obvious effects of philosophical inquiries; sometimes extending our views to objects which are removed from vulgar observation; and sometimes correcting our first apprehenfions with respect 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 disposed to consider it, as a fact which does not refult from will, but from necessity. To such men, it may be useful to direct their attention to the universal law of gravitation; which, although not more wonderful in itself, than the common effects of impulse, is more fitted, by its novelty, to awaken their

• attention, and to excite their curiofity. If the theo. ry of Boscovich should ever be established on a fatisfactory foundation, it would have this tendency in a still more remarkable degree, by teaching us that the communication of motion by impulse, (which we are apt to confider as a necessary 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 most remote parts of the universe.

If such, however, be the effects of our philofophical pursuits when fuccessfully conducted, it must be confefsed that the tendency of imperfect or erroneous theories is widely different. By a specious folution of infuperable difficulties, they so 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 exiftence of a region beyond it, into which philofophy is not permitted to enter. In such cafes, it is the business of genuine science to unmask the impofture, and to point out clearly, both to the learned and to the vulgar, what reason can, and what the cannot, accomplish. This, I apprehend, has been done, with respect to the history of our perceptions, in the most fatisfactory manner, by Dr. Reid.When a perfon little accustomed to metaphyfical fpeculations is told, that, in the case of volition, there are certain invisible 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 species, or phantasms, or images, which are present to the mind in the sensorium; he is apt to conclude, that the intercourse between mind and matter is much less mysterious than he had supposed; and that, although these expreffions may not convey to him any very distinct meaning, their import is perfectly understood by philosophers. It is now, I think, pretty generally acknowledged by physiologifts, 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 aside all the common hypothetical language concerning perception, and to exhibit the difficulty in all its magnitude, by a plain statement of the fact. To what then, it may be asked, does this statement amount?-Merely to this; that the mind is fo formed, that certain impressions produced on our organs of sense by external objects, are followed by correspondent sensations; and that these sensations, (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 process are equally incomprehensible; and that, for any thing we can prove to the contrary, the connexion between the sensation and the perception, as well as that between the impreffion and the sensation, may be both arbitrary : that it is therefore by no means impossible, that our sensations may be merely the occafions on which the correspondent 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 constitution of our nature, certain sensations are rendered the constant antecedents of our perceptions, it is just as difficult to explain how our perceptions are obtained by their means, as it would be, upon the supposition, that the mind were all at once inspired with them, without any concomitant sensations whatever.

These remarks are general, and apply to all our various perceptions; and they evidently strike at the root of all the common theories upon the fubject. The laws, however, which regulate these perceptions, are different in the cafe of the different fenses, and form a very curious object of philofophical inquiry. Those, in particular, which regulate the acquired perceptions of fight, lead to some very interesting and important speculations; 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 shall have occasion to make a few remarks on them, in the chapter on Conception.

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In opposition to what I have here observed on the importance of Dr. Reid's speculations concerning our perceptive powers, I am sensible it may be urged, that they amount merely to a negative discovery; and it is possible, that some may even be forward to remark, that it was unnecessary to employ so much labor and ingenuity as he has done, to overthrow. an hypothesis of which a plain account would have been a fufficient refutation. To such perfons, I would beg leave to suggest, that, although, in confequence of the juster views in pneumatology, which now begin to prevail, (chiefly, I believe, in confe quence of Dr. Reid's writings,) the ideal system 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 positive discoveries, in the whole history of science, can be mentioned, which found a juster claim to literary reputation, than to have detected, so clearly and unanswerably, the fallacy of an hypothesis, which has descended to us from the earliest ages of philofophy: and which, in modern times, has not only served to Berkeley and Hume as the basis of their sceptical systems, 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 exists in the mind fome object diftinct from the thinking substance were naturally led to inquire whence these 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 respect to this question, the opinions of the ancients were various; but as the influence of thefe opinions on the prevailing systems of the present age is not very confiderable, it is not necessary, 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 subject; some 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 sensation and reflection; and many, (especially among the later metaphysicians in France,) that they may be all traced from fenfation alone.

Of these theories, that of Mr. Locke deserves more particularly our attention; as it has ferved as the basis of most of the metaphysical systems which have appeared fince his time; and as the difference between it and the theory which derives all our ideas from sensation alone, is rather apparent than real.

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

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