compleat fyftem indeed was not to be expected from one man, or one age, or perhaps from the greateit number of ages; could we have expected it from the abilities of any one man, we furely should have had it from Sir Ifaac Newton: but he faw too far into nature to attempt it. How far he has carried this work, and what are the most important of his discoveries, we now proceed to confider.

воок II.

Of the theory of motion, or rational mechanics.

CHAP. I.

Of Space, time, matter, and motion.

"A

S we are certain of our own existence, and of that of our ideas, by internal conscioufnefs; fo we are satisfied, by the fame consciousness, that there are objects, powers, or causes without us, and that act upon us. For in many of our ideas, particularly thofe that are accompanied with pain, the mind must be paffive, and receive the impreffions (which are involuntary) from external caufes or inftruments, that depend not upon us. We easily distinguish thefe objects into two general claffes. The firft is of thofe which we perceive to have a spontaneity, or felf-moving power, and several properties and affections fimilar to thofe of our own minds, fuch as reafoning, judging, willing, loving, hating, &c. The fecond general clafs is of those in which no fuch affections appear, but which are so far of a paffive nature, that they never move of themselves, neither, when they are in motion, do they ever flop without fome external influence. If one of these move out of its place, without the appearance of a mover, we immediately conclude that this is owing to fome invifible agent; fo much are we perfuaded of its own inertia. If we lay up one of them in any place, we expect to find it there at

any

any distance of time, if no other powers have had access to it. This paffive nature, or inertia, is what chiefly distinguishes the fecond clafs of external objects, which is called body or matter; as the former is called mind or spirit.

2. How external objects, of either clafs, act upon the mind, by producing fo great a variety of impreffions or ideas, is not our business at present to enquire: neither is it neceffary for us to determine how exact or perfect the refemblance may be between our ideas and the objects or fubftances they represent. In our ideas which are repetitions of other ideas, we find very different degrees of refemblance between them and thofe of which they are repetitions. The idea we form in our imagination of a perfon, place, or figure which we have often feen, has a much more perfect resemblance to the impreffion we receive from fenfe, than the idea we are able in our imagination to form of pain, as to the fenfation we have felt of it. And as it is no objection against the existence of the fouls of other men, that they may be very different from the notion or conception we may have formed of them; fo it is no juft reafon against admitting the existence of body, that its inward effence, or fubftratum, may be very different from any thing we know of it. It is, however, rating our ideas of external objects by much too low, to compare them to words or arbitrary figns, ferving only to diftinguish them from each other. For it is from our ideas of them that we learn their properties, relations, and their influences upon each other, and upon our minds and those of others, and acquire useful knowledge concerning them and ourselves. For example, by comparing and examining our ideas, we judge of order and confufion, beauty and deformity, fitnefs and

H 4

unfit

unfitness, in things. The ideas of number and proportion, upon which fo ufeful and extenfive fciences are founded, have the fame origin.

3. The mind is intimately confcious of its own activity in reflecting upon its ideas, in examining and ranging them, in forming fuch as are complex from the more fimple, in reafoning from them, and in its elections and determinations. From this, as well as from the influence of external objects upon the mind, and from the course of nature, it easily acquires the ideas of caufe and effect. When a figure described upon a board produces a fimilar idea or impreffion on all thofe who fee it, it is as natural to afcribe this to one caufe, as, when we speak to a numerous audience, the effect of the difcourfe is to be afcribed to us; tho' we may be unable to explain how the impreffion of the figure is communicated to the feveral fpectators, or the difcourfe to the hearers. It were eafy to make many more remarks on the philofophy of those whofe principles would lead them. to maintain, that external objects vary with our preceptions, and that the object is always different when perceived by different minds, or by the fame perfon at different times, or in different circumftances. It will not be expected from us that we should enter farther, in a treatife of this kind, into the examination of doctrines as fruitless as they are extravagant.

4. Body not only never changes its state of itself, in confequence of its paffive nature or inertia, but it alfo refifts when any fuch change is produced: when at reft, it is not put in motion without difficulty; and when in motion, it requires a certain force to ftop it. This force with which it endeavours to perfevere in its ftate, and refifts any change, is called its vis inertiæ; and arifes from the inertia

of

of its parts, being always proportional to the quantity of matter in the body; infomuch that it is by this inertia only we are able to judge of the quantity of matter. And this judgment is well founded, because we constantly find that when we double or triple a body, or increafe or diminish it in any proportion, we must double or triple the force that is requifite to move it with the fame celerity, or increase or diminish it in the fame proportion with the body. If the folid, uncompounded particles void of pores, of equal bulk, have their inertia equal, then this must be accurately true: but if matter be of kinds fo different from each other, that the folid elementary particles of the one have a greater inertia than equal folid elementary particles of the other kind, then it is only when we compare thofe of the fame kind, that we can affirm the inertia to be proportional to the quantity of matter. Such different kinds of matter may exift for ought we know; but it is by diminishing or increafing the number or dimenfions of the pores of bodies that they are condenfed or rarified, according to our experience, and thereby the inertia of a given bulk increased or diminished.

5. Space is extended without limits, immoveable, uniform and fimilar in all its parts, and void of all resistance. It confifts indeed of parts which may be distinguished into other parts, lefs and lefs, without end, but cannot be feparated from each other, and have their fituation and distances changed.

6. Body is extended in space, moveable, bounded by figure, folid, and impenetrable, refifting by its inertia, divisible into parts, lefs and lefs, without end, that may be separated from each other and have their fituation or distances changed in any manner.