nam libertatis ejurationem, perpetuamque ingenii fervitutem.

But though I take the liberty to differ materially from you on certain points in philofophy, I embrace with joy this opportunity, the beft or only I may ever have, of acknowledging what pleasure and inftruction I have derived from your conversation and writings; and of expreffing the high fenfe that I have of the merit of your philofophy; which hath rescued one of the noblest and most interesting branches of knowledge, that of the Human Mind, from a state that rendered it unworthy of the name of Science, and a reproach to the human understanding; an object of contempt to the Wife, of deteftation to the Good, and of well-merited derifion even to the Vulgar.

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That fyftem of falfe fcience, which have fo happily exploded, as dangerous to

the

the best interests of mankind, has already undergone the feverest revision and cenfure of fome refpectable authors, who, being themselves warmly interested in the cause of religion and virtue, were disposed to regard with the most jealous abhorrence every thing, and every word, that tended, however remotely, or threatened, however feebly, to shake their facred foundations.

But its baneful effects on science itself have been greater and more evident; not merely by the many falfe and extravagant opinions to which it led, for these of themfelves must foon have died away, but by poisoning the very fountain of knowledge; introducing the most vifionary principles as fundamental truths, with a perfect neglect and contempt of that patient impartial observation, and of that strict and cautious induction, which alone can lead to the real knowledge of nature. It has thus

tended

tended rather to bewilder and darken mankind, than to lead and enlighten them in the paths of fcience; and has too often made its votaries rather ingenious disputants than folid reafoners. Nor has it been lefs pernicious, by deterring the ablest and most ardent in the purfuit of knowledge, with the unpromising appearance which it presented, and the unfatisfactory attainments which it afforded; by fatisfying the indolent with very imperfect and falfe accounts of those vaft regions of science, which, by proper exertions of their own, they might well have hoped to explore; and by filling the vain, for even in science vanity bears sway, with an arrogant conceit of fuperior knowledge, which they had neither talents nor perseverance to attain.

I must ever be of opinion, that those authors have deferved well of mankind, who have contributed by their labours, either

4

either to make men wifer, or to make them better; but they furely deserve doubly well of them who do both. That fuch is the happy spirit and tendency of your philofophy, and that fuch will be the influence of your writings on all who shall perufe them with due attention, and with out prejudice, I am well convinced. This profpect, and the affurance that you do not labour in vain, while the pure funfhine of a well-fpent life gilds the evening of your days, will, I hope, encourage you ftill to prosecute those inquiries, in which have hitherto been fo fuccefsful.

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INTRODUCTION.

T must no doubt appear very strange,

IT

and perhaps will be thought not very wife, for one, whofe profeffion and station engage him in the cultivation of physical science, ever to wander from his own more pleasant and fruitful province, into the thorny and barren regions of metaphysical fpeculation.

Such a conduct it is perhaps impoffible fully to justify, and it may be difficult even to excufe; but in the prefent instance it may be, and it furely ought to be, accounted for at least.

The treatife contained in this volume is part of a much more extenfive and important undertaking, An Essay towards an investigation of the exact import and extent of the common notion of the relation of Caufe