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acts, nonfenfe and confufion muft of neceffienfue; fcience will foon come to have neither head nor tail, beginning nor end; philofophy will grow contemptible; and its adherents, far from being treated, as in former times, upon the footing of conjurers, will be thought by the vulgar, and by every man of fenfe, to be little better than downright fools.






UT now a difficulty occurs, which it is not eafy to folve. Granting what is faid above to be true; that all legitimate reafoning, whether of certain or of probable evidence, does finally refolve itself into principles of common fenfe, which we must admit as certain, or as probable, upon their own authority; that therefore common fenfe is the foundation and the ftandard of all just reasoning; and that the genuine fentiments of nature are never erroneous: yet, by what criterion fhall we know a fentiment of nature from a prejudice of education, a dictate of common fenfe from the fallacy of an inveterate opinion? Muft every principle be admitted as true, which we believe without being able to affign a reason ? then where is our fecurity against prejudice and implicit faith! Or muft every principle that seems intuitively certain, or intuitively probable, be reafoned upon, that we may know whether it be really what it feems? then

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then where our fecurity against the abuse fo much infifted on, of subjecting common sense to the test of reasoning! -At what point must reason stop in its investigations, and the dictates of common fenfe be admitted as decifive and final?

It is much to be regretted, that this matter has been fo little attended to: for a full and fatisfactory difcuffion of it would do more real service to the philofophy of human nature, than all the fyftems of logic in the world; would at once exalt pneumatology to the dignity of fcience, by fettling it on a firm and unchangeable foundation; and would go a great way to banish sophistry from fcience, and rid the world of fcepticism. This is indeed the grand defideratum in logic; of no lefs importance to the moral fciences, than the difcovery of the longitude to navigation. That I fhall fully folve this difficulty, I am not fo vain, nor fo ignorant, as to imagine. But I humbly hope I fhall be able to throw fome light on the fubject, and contribute a little to facilitate the grefs of those who may hereafter engage in the fame purfuit. If I can accomplish even this, I fhall do a fervice to truth, philofophy, and mankind: if I fhould be thought to fail, there is yet fomething meritorious in the attempt. To have fet the example, may be of confequence.


I fhall endeavour to conduct the reader to the conclusion I have come to on this subject,


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by the fame steps that led me thither; a method which I prefume will be more perfpicuous, and more fatisfying, than if I were first to lay down a theory, and then affign the reafons. By the way, I cannot help expreffing a wifh, that this method of inveftigation were lefs uncommon, and that philofophers would fometimes explain to us, not only their discoveries, but also the process of thought and experiment, whether accidental or intentional, by which they were led to them.

If the boundary of Reafon and Common Senfe had never been fettled in any fcience, I would abandon my present scheme as defperate. But when I reflect, that in fome of the fciences it has been long fettled, with the utmost accuracy, and to univerfal fatiffaction, I conceive better hopes; and flatter myself, that it may perhaps be possible to fix it even in the philofophy of the mind. The fciences in which this boundary has been long fettled and acknowledged, are, mathematics, and natural philofophy; and it is remarkable, that more truth has been difcovered in thofe fciences than in any other. Now, there is not a more effectual way of learning the rules of any art, than by attending to the practice of those who have performed in it moft fuccefsfully: a maxim which, I fuppofe, is no lefs applicable to the art of investigating truth, than to the mechanical and the fine arts. Let us fee, then, whether,

whether, by attending to the practice of mathematicians and natural philofophers, as contrafted with the practice of those who have treated of the human mind, we can make any discoveries preparatory to the folution of this difficulty.


Confirmation of this theory from the practice of Mathematicians and Natural Philofophers.



HAT the diftinction between Reafon and Common Senfe, as here explained, is acknowledged by mathematicians, we have already shown*. They have been wife enough to trust to the dictates of common fenfe, and to take that for truth which they were under a neceffity of believing, even though it was not in their power to prove it by argument. When a mathematician arrives, in the course of his reasoning, at a principle which he must

* See part 1. chap. 2. fect. 1.




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