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the unknown to all the other known fpecies of the fame genus, and thence to all the known ge nera of the fame order or tribe; and having experienced in the one infiance, à regularity ́ in every fpecies, genus, and tribe, which admits no exception; in the other, a variety as boundlefs as is that of season, soil, and culture; it learns hence to mark the difference.

AGAIN, we may observe, that, on a clofer acquaintance with thofe objects wherewith we are furrounded, we come to difcover that they are mostly of a compound nature, and that not only as containing a complication of those qualities called accidents, as gravity, mobility, colour, extenfion, figure, folidity, which are common almost to all matter, not only as confifting of different members, but as comprehending a mixture of bodies, often very different in their nature and properties, as air, fire, water, earth, falt, oil, fpirit, and the like. Thefe, perhaps, on deeper researches, will be found to confift of materials ftill fimpler. Moreover, as we advance in the ftudy of Nature, we daily find more reafon to be convinced of her conftancy in all her operations, that like causes in like circumftances always produce like effects, and inverfely like

effects

effects always flow from like causes. The inconftancy which appears at first in fome of Nature's works, a more improved experience teacheth us to account for in this manner. As most of the objects we know, are of a complex nature, on a narrower fcrutiny we find, that the effects ascribed to them, ought often folely to be afcribed to one or more of the component parts; that the other parts noway contribute to the production; that, on the contrary, they fometimes tend to hin der it. If the parts in the compofition of fimilar objects were always in equal quantity, their be ing compounded would make no odds; if the parts, though not equal, bore always the fame proportion to the whole, this would make a dif ference; but fuch as in many cases might bẹ computed. In both refpects, however, there is an immenfe variety. Perhaps every individual differs from every other individual of the fame fpecies, both in the quantities and in the proportions of its conftituent members and component parts. This diverfity is alfo found in other things, which, though hardly reducible to fpecies, are generally known by the fame name. The atmosphere in the fame place at different times, or at the fame time in different places, differs in denfity, heat, humidity, and the num

ber,

ber, 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 afcertained by a general experience of the uniformity of their operations. And though perhaps it be impoffible for us to attain the knowledge of the fimpleft elements of any body, yet, when any thing appears fo fimple, or rather fo exactly uniform, as that we have obferved it invariably to produce fimilar effects; on discovering any new effect, though but by one experiment, we conclude from the general experience of the efficient, a like conftancy in this energy as in the reft. Fire confumes wood, melts copper, and hardens clay. In thefe inftances it acts uniformly, but not in these only. I have always experienced hitherto, that whatever of any fpecies is confunied by it once, all of the fame fpecies it will confume upon trial at any time. The like may be faid of what is melted, or hardened, or otherwife altered, by it. If then, for the first time, I try the influence of fire on any foffil, or other fubftance; whatever be the effect, I readily conclude, that fire will always produce á fimilar effect on fimilar bodies. This conclusion is not founded on this fingle instance, but on this inftance compared with a general experience

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perience 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, befides, the principal organ of truth in all the branches of phyfiology, (I ufe the word in its largest acceptation,) including natural history. aftronomy, geography, mechanics, optics, hydroftatics, meteorology, medicine, chymiftry. Under the general term I alfo comprehend natural theology and pfychology, which, in my opinion, have been moft unnaturally disjoined by philo fophers. Spirit, which here comprises only the Supreme Being and the human foul, is furely. as much included under the notion of natural object, as body is, and is knowable to the philofopher purely in the fame way, by obfervation and experience.

II. Analogy.

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THE evidence of analogy, as was hinted above, is but a more indirect experience, founded on fome remote fimilitude. As things, however, are often more eafily comprehended by the aid of

example,

example, than by definition, I thall in that manner illuftrate the difference between experimental evidence, and analogical. The circulation of the blood in one human body is, I fhall suppose, experimentally difcovered. Nobody will doubt of this being a fufficient proof from experience, that the blood circulates in every human body? Nay further, when we confider the great fimila rity which other animal bodies bear to the human body, and that both in the ftructure and in the deftination of the feveral organs and limbs; particularly, when we confider the 'refemblance in the blood itself, and blood veffels, and in the fabric and pulfation of the heart and arteries, it will appear fufficient experimental evidence of the circulation of the blood in brutes, especially in quadrupeds. Yet, in this application, it is manifeft, that the evidence is weaker than in the former. But should I from the fame experiment infer the circulation of the fap in vegetables, this would be called an argument only from analogy. Now all reafonings from experience are obviously weakened in proportion to the remotenefs of the refemblance fubfifting between that on which the argument is founded, and that concerning which we form the conclufion.

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