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Sect. II. Of deductive evidence.
count for in this manner. As most of the objects we know, are of a complex nature, on a narrower scrutiny we find, that the effects ascribed to them, ought often solely to be ascribed to one or more of the component parts; that the other parts noway contribute to the production; that, on the contrary, they sometimes tend to hinder it. If the parts in the composition of similar objects were always in equal quantity, their being compounded would make no odds; if the parts, though not equal, bore always the same proportion to the whole, this would make a difference; but such as in many cases might be computed. In both respects, however, there is an immense variety. Perhaps every individual differs from every other indivi- . dual of the same species, both in the quantities and in the proportions of its constituent members and com- . ponent parts. This diversity is also found in other things, which, though hardly reducible to species, are generally known by the same name. The atmosphere; in the same place, at different times, or at the same time in different places, differs in density, heat, humidity, and the number, quality, and proportion of the , vapours or particles with which it is loaden. The more then we become acquainted with elementary natures, the more we are ascertained by a general experience of the uniformity of their operations. And though perhaps it be impossible for us to attain the knowledge of the simplest elements of any body, yet, when any thing appears so simple, or rather so exactly uniform, as that we have observed it invariably to produce similar effects; on discovering any new effect,
Part III. The subdivisions of moral reasoning.....l. Experience.
though but by one experiment, we conclude, from the general experience of the efficient, a like constancy in this energy as in the rest. Fire consumes wood, melts copper, and hardens clay. In these instances it acts uniformly, but not in these only. I have always experienced hitherto, that whatever of any species is
consumed by it once, all of the same species it will
consume upon trial at any time. The like may be
said of what is melted, or hardened, or otherwise altered, by it. If then, for the first time, I try the in
fluence of fire on any fossil, or other substance; whatever be the effect, I readily conclude, that fire will always produce a similar effect on similar bodies. This conclusion is not founded on this single instance, but on this instance compared with a general experience of the regularity of this element in all its operations. - - * *.*, So much for the first tribe, the evidence of experience, on which I have enlarged the more, as it is, if not the foundation, at least the criterion of all moral
reasoning whatever. It is, besides, the principal or
gan of truth in all the branches of physiology, (I use
the word in its largest acceptation) including natural
history, astronomy, geography, mechanics, optics, hydrostatics, meteorology, medicine, chymistry. Under
the general term I also comprehend natural theology :
and psychology, which, in my opinion, have been most unnaturally disjoined by philosophers. Spirit, which here comprises only the Supreme Being and the human soul, is surely as much included under the notion of natural object, as body is, and is knowable to the
Sect. II. Of deductive evidence.
philosopher purely in the same way, by observation and experience,
The evidence of analogy, as was hinted above, is but a more indirect experience, founded on some remote similitude. As things, however, are often more easily comprehended by the aid of example, than by definition, Ishall in that manner illustrate the difference between experimental evidence, and analogical. The circulation of the blood in one human body, is, I shall suppose, experimentally discovered. Nobody will doubt of this being a sufficient proof from experience, that the blood circulates in every human body. Nay, further, when we consider the great similarity which other animal bodies bear to the human body, and that both in the structure and in the destination of several organs and limbs; particularly when we consider the resemblance in the blood itself, and blood vessels, and in the fabric and pulsation of the heart and arteries, it will appear sufficient experimental evidence of the
circulation of the blood in brutes, especially in qua
drupeds. Yet, in this application, it is manifest, that the evidence is weaker than in the former. But should I from the same experiment infer the circulation of the sap in vegetables, this would be called an argument only from analogy. Now all reasonings from experience are obviously weakened in proportion to the remoteness of the resemblance subsistin g between that
Part III. The subdivisions of moral reasoning.'... II. Analogy.
on which the argument is founded, and that concerning which we form the conclusion,
The same thing may be considered in a different way. I have learned from experience, that like effects sometimes proceed from objects which faintly resemble, but not near so frequently as from objects which have a more perfect likeness. By this experience I am enabled to determine the degrees of probability from the degrees of similarity, in the different cases. It is presumeable that the former of these ways has the earliest influence, when the mind, unaccustomed to reflection, forms but a weak association, and consequently but a weak expectation, of a similar event from a weak resemblance. The latter seems more the result of thought, and is better adapted to the ordinary forms of reasoning,
It must be allowed, that analogical evidence is at best but a feeble support, and is hardly ever honoured with the name of a proof. Nevertheless, when the analogies are numerous, and the subject admits not evidence of another kind, it doth not want its ef. ficacy. It must be owned, however, that it is generally more successful in silencing objections, than in evincing truth, and on this account may more properly be styled the defensive arms of the orator, than the offensive. Though it rarely refutes, it frequently repels refutation, like those weapons which, though they cannot kill the enemy, will ward his blows ".
Sect. II. Of deductive evidence.
The third tribe is the evidence of testimony, which is either oral or written. This also hath been thought by some, but unjustly, to be solely and originally derived from the same source, experience f. The utmost, in regard to this, that can be affirmed with truth, is, that the evidence of testimony is to be considered as strictly logical, no farther than human veracity in general, or the veracity of witnesses of such a character, and in such circumstances in particular, is supported; or, perhaps more properly, hath not been refuted by experience. But that testimony, antecedently to experience, hath a natural influence on belief, is undeniable. In this it resembles memory; for though the defects and misrepresentations of memory are corrected by experience, yet that this faculty hath an innate evidence of its own, we know from this; that if we had not previously given an implicit faith to memory, we had never been able to acquire experience. This will appear from a revisal of its nature, as explained above. Nay, it must be owned, that in what regards single facts, testimony is more adequate evidence than any conclusions from experience. The imme
Jigion natural and revealed, to the constitution and course of nature, hath shewn us, how useful this mode of reasoning may be rendered, by the application he hath so successfully made of it, for refuting the cavils of infidelity. + I had occasion to make some reflections on this subject formerly, See Dissertation on Miracles, Part I. Sect. I. There are several ingenious observations on the same subject in Reid's Inquiry.
Ch. VI. Sect. XXIII. -