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TI1E NATURE AND CAUSES OF

Different authors gave different systems both of natural andmoral philosophy. But the arguments by ■which they sup-ported those different systems, far from being always demon-strations, were frequently at best but very slender probabili-ties, and sometimes mere sophisms, which had no otherfoundation but the inaccuracy and ambiguity of commonlanguage. Speculative systems have in all ages of the worldbeen adopted for reasons too frivolous to have determined thejudgment of any man of common sense, in a matter of thesmallest pecuniary interest. Gross sophistry has scarce everhad any influence upon the opinions of mankind, except inmatters of philosophy and speculation ; and in these it hasfrequently had the greatest. The patrons of each system ofnatural and moral philosophy naturally endeavoured to ex-pose the weakness of the arguments adduced to support thesystems which were opposite to their own. In examiningthose arguments, they were necessarily led to consider thedifference between a probable and a demonstrative argu-ment, between a fallacious and a conclusive one ; and Logic,or the science of the general principles of good and badreasoning, necessarily arose out of the observations which ascrutiny of this kind gave occasion to. Though in its origin,posterior both to physics and to ethics, it was commonlytaught, not indeed in all, but in the greater part of the an-cient schools of philosophy, previously to either of thosesciences. The student, it seems to have been thought, oughtto understand well the difference between good and badreasoning, before he was led to reason upon subjects of sogreat importance.

This ancient division of philosophy into three parts was,in the greater part of the universities of Europe , changed foranother into five.

In the ancient philosophy, whatever was taught concerningthe nature either of the human mind or of the Deity, made apart of the system of physics. Those beings, in whatevertheir essence might be supposed to consist, were parts of thegreat system of the universe, and parts too productive of themost important effects. Whatever human reason could eitherconclude, or conjecture, concerning them, made, as it were,two chapters, though no doubt two very important ones, ofthe science which pretended to give an account of the originand revolutions of the great system of the universe. But inthe universities of Europe , where philosophy was taught