Sivut kuvina
PDF
ePub

proof. This, for order's fake, I have diftributed into three claffes, the truths of pure intellection, of consciousness, and of common fenfe. The first may be denominated metaphyfical, the fecond phyfical, the third moral; all of them natural, original, and unaccountable.

SECTION II.

Of deductive evidence.

.it

PART I. Divifion of the fubject into Scientific and moral, with the principal diftinctions between them.

ALL rational or deductive evidence is derived from one or other of thefe two fources: from the invariable properties or relations of general ideas; or from the actual, though perhaps, variable connexions, fubfifting among things. The former we call demonftrative, the latter moral. Demonftration is built on pure intellection, and confifteth in an uninterrupted series of axioms. That propofitions formerly demonftrated are taken into the feries, doth not in the leaft invalidate this account; inafmuch as thefe propofitions are all refolvable into axioms, and are admitted as links in the chain;

not

not becaufe neceffary, but merely to avoid the ufelefs prolixity which frequent and tedious repetitions of proofs formerly given would occafion. Moral evidence is founded on the principles we have from confcioufnefs and common fenfe, improved by experience; and as it proceeds on this general prefumption or moral axiom, that the courfe of nature in time to come, will be fimilar to what it hath been hitherto, it decides, in regard to particulars, concerning the future from the paft, and concerning things unknown, from things familiar to us. The first is folely converfant about number and extenfion, and about those other qualities which are meafurable by these. Such are duration, velocity, and weight. With regard to fuch qualities as pleasure and pain, virtue and vice, wifdom and folly, beauty and deformity, though they admit degrees, yet, as there is no ftandard or common measure, by which their differences and proportions can be afcertained and expreffed in numbers, they can never become the fubject of demonftrative reafoning. Here rhetoric, it must be acknowledged, hath little to do. Simplicity of diction, and precifion in arrangement, whence refults perfpicuity, are, as was obferved already *, Chap. I. Part 1.

all

all the requifites. The proper province of rhetoric is the second, or moral evidence; for to the second belong all decifions concerning fact, and things without us.

BUT that the nature of moral evidence may be better understood, it will not be amifs to remark a few of the moft eminent differences be tween this and the demonstrative,

THE firft difference that occurs is in their fubjects. The fubject of the one is, as hath been obferyed, abftract independent truth, or the unchangeable and neceffary relations of ideas; that of the other,the real, but often changeable and contingent connexions that fubfift among things actually exifting. Abstract truths, as the properties of quan quan, tity, have no refpect to time or to place, no dependence on the volition of any being, or on any cause whatever, but are eternally and immutably the fame. The very reverse of all this generally obtains with regard to fact. In confequence of what has been now advanced, affertions oppofite to truths of the former kind, are not only falfe, but abfurd. They are not only not true, but it is impoffible they should be true, whilft the meanings of the words (and confequently the ideas compared) remain the fame. This doth not hold com

monly

monly in any other kind of evidence. Take, for inftance, of the first kind, the following af firmations, The cube of two is the half of fix

[ocr errors]
[ocr errors]
[ocr errors]

teen.' The fquare of the hypotenufe is equal to

،

the fum of the squares of the fides.' If equal things be taken from equal things, the remainders will be equal.' Contrary propofitions, as, The cube of two is more than the half of fixteen.' 'The fquare of the hypotenufe is lefs than the fum of the squares of the fides.' If equal things be taken from equal things, the remainders will be unequal,' are chargeable, not only with falfity, but with abfurdity, being inconceivable and contradictory. Whereas, to these truths which we acquire by moral evidence, Cæfar overcame ⚫ Pompey.' The fun will rife to-morrow."

All men will die,' the oppofite affertions, though untrue, are eafily conceivable without changing, in the leaft, the import of the words, and therefore do not imply a contradiction,

THE fecond difference I fhall remark is, that moral evidence admits degrees, demonftration doth not. This is a plain confequence of the preceding difference. Effential or neceffary truth, the fole object of the latter, is incompatible with degree. And though actual truth, or matter of

fact,

fact, be the ultimate aim of the former, likeli hood alone, which is fufceptible of degree, is ufually the utmost attainment. Whatever is exhibited as demonftration, is either mere illufion, and fo no evidence at all, or abfolutely perfect. There is no medium. In moral reafoning we afcend from poffibility, by an infenfible gradation, to probability, and thence, in the fame manner, to the fummit of moral certainty. On this fummit, or on any of the steps leading to it, the conclufion of the argument may reft. Hence the result of that is, by way of eminence, denominated science; and the evidence itself is termed scientific; the refult of this is frequently (not always) intitled to no higher denomination. than opinion. Now, in the mathematical fciencés, no mention is ever made of opinions.

THE third difference is, that in the one there never can be any contrariety of proofs; in the other, there not only may be, but almost always is. If one demonftration were ever capable of being refuted, it could be folely by another demonftration, this being the only fort of evidence adapted to the fubject, and the only fort by which the former could be matched. But, to suppose that contraries are demonftrable,

[ocr errors]
« EdellinenJatka »