With respect to the degree of evidence which ought to be expected in natural philosophy, it is proper to remark, that physical matters are not, in general, capable of such absolute certainty as the branches of mathematics. The propositions of the latter science are clearly deduced from a set of axioms so very simple and evident, as to convey perfect conviction to the mind ; nor can any of them be denied without a mani fest absurdity. But in natural philosophy we can only say, that because certain particular effects have been constantly produced under certain circumstances, therefore they will most probably continue to be produced as long a-s the same circumstances exist ; and likewise that they do, in all pro bability, depend upon those circumstances. And this is what we lens to by of nuture, viz.. certain effects which are, or have been uniforn4, produced by certain causes, as flu as our observations reach.
We may, indeed, assume various physical principles, and by reasoning upon them, we may strictly demonstrate the deduction of certain consequences. But as the demonstration goes no farther than to prove, that such consequences must iweessarily folio NV the principles W hick have been assumed ; the eonsequenues themselves can have no greater degree of certainty than the principle: are possessed of; so that they are true, or filse, or according as the principles upon which they depend are true, or fills?, or probable.
The tbund:itions of experimental philosf pity, as we have already observed, arc the properties of natural bodies, viz., of all these bodies, either solid or 11111.1, which in any way :iffect any of our senses ; and sinve our senses are affected by the properties of these bodies, viz., by their extension, colour. hardness, transparency, &c.. we cannot know any more of these bodies than what is Inallifested to us by such properties only as we are able to perceive. Were we furnished with other senses, donbtless we Might discover other properties which would make us more intimately acquainted with the such bodies.
Human art has not been able to discover more senses than those which everybody knows; but it has. in great measure, improved some or those which we possess, and this alone is sufficient to pniut out the limited nature of our per ceptions. Thus. tb• instance, the discovery of the micro scope and the telescope have shown us wonders, of which our forefiithers were utterly ignorant ; and the number and variety of these wonders have increased, in proportion as the above-mentioned instruments have been improved. The
Improvements of these instruments have been suggested by the discoveries that have been made respecting the refrangi bility of fight, and the properties of transparent bodies, and these have been Inade i11 eensequence of the innumerable experiments that have been instituted by various intelligent persons. Thus It appears, that by means of trials and obser vations, new facts are ascertained, which, besides their beimg immedratety usetul to the human species, furnish, at the same time, the means of making farther discoveries; and the trea sures of the natural world are fi•, indeed, from a state of exhaustion. Hence the improvements and the discoveries of experimental philosophy proceed in a kind of increasing geometrical progression ; unless they are impeded by some extraordinary occurrence.
In Contemplating the intimate nature of natural bodies, when our mind goes beyond the bounds of our senses. (and our senses, even with the assistance of instruments and rea soning, are only capable of perceiving a few properties of those we wander in the boundless field of probability and conjecture. Two principal hypotheses have been enter tained with respect to the primitive compenent particles of bodies. One is, that the particles of each peculiar species of bodies are different from the particles of another species of bodies. Thus the primitive particles of geld are supposed to be different from the particles of calearerms earth. different from the particles of water. &e. The other hypothesis is, that there is one kind or primitive. or original particles of !natte•, and that from the different arrange M ent of those ultimate particles, the various bodies :wise. Experience shows, that Certain bodies, which at first sight appear to be abso lutely different from each other, are, upon farther exami nation, exactly of the same nature. On tile ether hand, a vast number of bodies are so distinct from each other. that no art has been able to f...•in 011C of them froth the particles of the other; thus gold cannot be converted into a diamond, iron cannot be converted into lead, The former of these observations seems to fliveur the s•cond hypothesis ; the latter seems to fit•our the first hypothesis; but it is not in our pow er to determine the real state of the matter.