fertile this period was in new inventions, nature did not unveil herself readily enough to fatisfy the impatience of fome men, who could not be contented with thofe views of her which time and inditry produced to them. Therefore they hear kned again to the vain promifes of thofe who pretended to unravel all her myfteries at once, by the force of their abftracted fpeculations. The Cartefian fyftem was the most extenfive, and (according to many, the moft exquifite in its contrivance, of any that have been imagined. The author of it was a bold philofopher, and doubtlefs of a fubtle genius, to indulge which he retired from the world for many years. He valued himself on his clear ideas, and is allowed to have contributed to diffipate the darkness of that fort of fcience which prevailed in the schools. If we may believe fome accounts, he rejected a void from a complaifance to the tafte which then prevailed, against his own first fentiments; and amongst his familiar friends, ufed to call his fyftem his philofophical romance. It had however great fuccefs; and his doctrines ftill prevail fo much, that it is neceffary for our purpose to give a fhort account of them.

CHA P. IV.

Of the philofophical principles of Des Cartes, the emendations of his followers, and the prefent controverfies in natural philofophy.

ES Cartes begins his principia by fhewing the

Dneceffity of doubting firft of every thing, in

order to our obtaining certain knowledge; and recommends to his readers to confider his reasons for doubting of all things, not once only, but to employ weeks, or even months, on these alone, before

he proceed farther. He first establishes the certainty of our own exiftence, and that of our ideas of which we are intimately confcious to ourselves; of the existence of which, however, after all he has faid, it feems impoffible for us to doubt for a moment. From our having the idea of a Being infinitely perfect and neceffarily exifting, he concludes that fuch a Being actually is; upon whofe will he makes the certainty of felf-evident propofitions, or axioms * as well as of all other neceffary truths, to depend.

From the knowledge of the caufe established in this manner, he pretends to deduce a complete knowledge of his effects, by neceffary fteps. It is clear, fays he †, that we fhall follow the beft method in philofophy, if, from our knowledge of the Deity himfelf, we endeavour to deduce an explication of all his works; that fo we may acquire the moft perfect kind of fcience, which is that of effects from their caufes. As for final caufes he rejected them from philofophy, as we obferved above; and from thefe paffages, which reprefent the genius of this author's philofophy, and from the manner in which he fets out, we may already form fome judgment how hopeful his project was.

From the veracity of the Deity, he infers the reality of material objects, which are reprefented to us as exifting without us. He places the effence of matter in extenfion; for this alone remains, he fays,

According to him, the Deity did not will that the three angles of a triangle fhould be equal to two right ones, becaufe he knew that it could not be otherwife; but, because he would that the three angles of a triangle fhould neceffarily be equal to two right ones, therefore this is true and can be no otherwife. + See the paffages cited above from his Principia, in the notes upon § 4. ch. 1.

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when

when we reject hardness, colour, weight, heat and cold, and the other qualities which, we know, a body can be without. Hence he easily concludes that there can be no void, or extension without matter. He adds, however, immediately afterwards, as properties of matter, that its parts are feparable and moveable; tho' thefe feem to imply more than mere extenfion.

He defines motion to be the translation of a body from the neighbourhood of other bodies that are in contact with it, and are confidered as quiefcent, to the neighbourhood of other bodies; and thus makes no diftinction between abfolute or real, and relative or apparent motions; both of which equally agree to this definition. The reafon he gives why the fame quantity of motion must be preferved for ever in the univerfe, without any augmentation or diminution in the whole, muft appear concife, and very extraordinary. It is no other than that God muft be fuppofed to act in the moft conftant and immutable manner. From the fame property of the Deity, he infers that a body must continue in its ftate as to reft, motion, figure, &c. till fome external influence produce a change; which is his first law of nature: that the direction of motion is naturally rectilinear, or that a body never changes its direction of itself; which is his fecond law: and that a body in motion, when it meets with another moving with a greater force, is reflected without lofing any part of its first motion; but when it meets with a body moving with lefs force, it then carries this body along, and lofes as much motion as is transferred to it; and this is his third law of nature. He accounts for the hardnefs of bodies from their parts being quiefcent with refpect to each other; and for fluidity, from their being moved perpetually in

all

all directions. He concludes the fecond part of his book with telling us, that thefe principles are fufficient for explaining all the phænomena of nature, and that no other ought to be admitted or even wifhed for.

He afterwards proceeds to fhew how the univerfe might have affumed its prefent form, and may be for ever preferved, by mechanical principles. He fuppofes the particles of matter to have been angular, fo as to replenish space without leaving any interstices between them; and to have been in perpetual agitations, by which the angular parts being broke off, the particles themfelves became round, and formed what he calls the matter of the fecond element. The angular parts, being ground into the moft fubtile particles of all, became the matter of his first element, and ferved to fill all the pores of the other. But there being more of this firft element than was neceffary for that purpose, it became accumulated in the centers of the vortices, of which he imagined the universe to confift, and formed there the bodies of the fun and ftars. The heavens were filled with the matter of the fecond element, the medium of light. But the planets and comets confifted of a third element groffer than the other two, the generation of which he traces at length through all its fteps. According to him, the matter of the firft element muft have conftantly flowed out through the interstices between the fpherical particles of the fecond element, where the circular motion is greateft, and must have returned continually at the poles of this motion towards the centre of the vortex; where being apt to cohere together, they at length produced the groffer particles of the third; and when these came to adhere in a confiderable quantity, they gave rife to the fpots on the furfaces of

F 3

the

the funs or ftars. Some being crufted over with fuch fpots became planets or comets; and the force of their rotation becoming languid, their vortices were abforbed by fome more potent neighbouring vortex. In this manner the folar fyftem was formed, the vortices of the fecondary planets having been abforbed by the vortex of the primary, and all of them by that of the fun. He contends that the parts of the folar vortex increase in denfity, but decreafe in celerity, to a certain diftance; beyond which he fuppofes all the particles to be equal in magnitude, but to increase in celerity as they are farther from the fun. In thofe upper regions of the vortex he places the comets; in the lower parts he ranges the planets; fuppofing thofe that are more rare to be nearer the fun, that they may correfpond to the denfity of the vortex where they are carried round.

He accounts for the gravity of terrestrial bodies from the centrifugal force of the æther revolving round the earth; which, he imagined, muft impell bodies downwards that have not fo great a centrifugal force, much in the fame manner as a fluid impells a body upwards that is immerged in it, and has a lefs fpecifical gravity than it. He pretended to explain the phænomena of the magnet, and to account for every thing in nature, from the fame principles.

2. There never was, perhaps, a more extravagant undertaking than fuch an attempt, to deduce, by neceffary confequences, the whole fabric of nature, and a full explication of her phænomena, from any ideas we are able to form of an infinitely perfect Being. Was it not for the high reputation of the author, and of his fyftem, it would be hardly excufable to make any remarks upon fuch a rhapfody.

Should