Different authors gave different systems both of natural and moral philosophy. But the arguments by which they supported those different systems, far from being always demonstrations, were frequently at best but very slender probabilities, and sometimes mere sophisms, which had no other foundation but the inaccuracy and ambiguity of common language. Speculative systems have in all ages of the world been adopted for reasons too frivolous to have determined the judgment of any man of common-sense, in a matter of the smallest pecuniary interest. Gross sophistry has scarce ever had any influence upon the opinions of mankind, except in matters of philosophy and speculation; and in these it has frequently had the greatest. The patrons of each system of natural and moral philosophy naturally endeavored to expose the weakness of the arguments adduced to support the systems which were opposite to their own. In examining those arguments they were necessarily led to consider the difference between a probable and a demonstrative argument, between a fallacious and a conclusive one; and Logic, or the science of the general principles of good and bad reasoning, necessarily arose out of the observations which a scrutiny of this kind gave occasion to. Though in its origin, posterior both to physics and to ethics, it was commonly taught, not indeed in all, but in the greater part of the ancient schools of philosophy, previously to either of those sciences. The student, it seems to have been thought, ought to understand well the difference between good and bad reasoning before he was led to reason upon subjects of so great importance.
This ancient division of philosophy into three parts was, in the greater part of the universities of Europe, changed for another into five.
