infuperable difficulties, they fo dazzle and bewilder the understanding, as, at once, to prevent us from advancing, with fteadiness, 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 cases, it is the business 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 fhe 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 accuftomed 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 apt to conclude, that the intercourfe between mind and matter is much less mysterious than he had fuppofed; and that, although thefe expreffions may not convey to him any very distinct meaning, their import is perfectly understood by philofophers. It is now, I think, pretty generally acknowledged by phyfiologifts, 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 firft perfon 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 statement of the fact. To what then, it may be asked, does this statement

statement amount?-Merely to this; that the mind is fo formed, that certain impreffions produced on our organs of sense 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 process 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 fenfations, 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 fubject, it follows, that it is external objects themfelves, and not any fpecies 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, yet it is just 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 what

ever.

These remarks are general, and apply to all our various perceptions; and they evidently ftrike at the

root

root of all the common theories upon the subject. The laws, however, which regulate these 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 per. ceptions of fight, lead to fome very interesting and important speculations; and, I think, have never yet been explained in a manner completely fatisfactory. 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 labour 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 suggest, that, although, in consequence of the juster views in pneumatology, which now begin to prevail, (chiefly, I believe, in confequence of Dr. Reid's writings,) the ideal system may appear to many readers un. philofophical and puerile; yet the cafe was very dif ferent when this author entered upon his inquiries; and I may even venture to add, that few pofitive dif coveries, 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 de

fcended

fcended 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 sceptical fyftems, but was adopted as an indisputable truth by Locke, by Clarke, and by Newton.

THE

SECTION IV.

Of the Origin of our Knowledge.

HE philofophers who endeavoured 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 diftin&t 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 form part of its original furniture?

With refpect to this queftion, the opinions of the ancients were various; but as the influence of thefe opinions on the prevailing fyftems 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, (especially among the later metaphyficians in France,) that they may be all traced from fenfation alone.

Of

Of these theories, that of Mr. Locke deferves more particularly our attention; as it has ferved as the bafis 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 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 fenfation, all the ideas which we are fuppofed to receive by the externa} fenfes; our ideas, for example, of colours, of founds, of hardness, of extenfion, 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 conscioufnefs; our ideas, for example, of memory, of imagination, of volition, of pleasure, and of pain. These two fources, according to him, furnifh 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 compofition, abstraction, generalisation, &c. on the materials which it thus collects in the courfe of its experience. The laudable defire of Mr. Locke, to introduce precifion and perfpicuity into metaphysical speculations, and his anxiety to guard the mind against error in general, naturally prepoffeffed him in favour of a doctrine, which, when compared with thofe of his predeceffors, was intelligible and fimple; and which, by fuggesting a method, apparently eafy and palpable, of analysing our knowledge into its elementary principles, feemed to furnish an antidote against those prejudices which had been favoured by the hypothefis of innate ideas. It is now

a con