On Logic (-1799)

"Among all the liberal arts, the first is logic, and specifically that part of logic which gives initial instruction about words. [...] the word 'logic' has a broad meaning, and is not restricted exclusively to the science of argumentative reasoning. [It includes] Grammar [which] is 'the science of speaking and writing correctly - the starting point of all liberal studies'." (John of Salisbury, "Metalogicon", 1159)

"Among the various branches of philosophy, logic has two prerogatives: it has both the honor of coming first and the distinction of serving as an efficacious instrument throughout the whole body [of philosophy]. Natural and moral philosophers can construct their principles only by the forms of proof supplied by logicians. Also, in order to define and divide correctly, they must borrow and employ the art of the logicians. And if, perchance, they succeed in this without logic, their success is due to luck, rather than to science. Logic is 'rational' [philosophy], and we may readily see from the very name, what progress in philosophy can be expected from one who [since he lacks logic] lacks reason." (John of Salisbury, "Metalogicon", 1159)

"The method of demonstration is therefore generally feeble and ineffective with regard to facts of nature (I refer to corporeal and changeable things). But it quickly recovers its strength when applied to the field of mathematics. For whatever it concludes in regard to such things as numbers, proportions and figures is indubitably true, and cannot be otherwise. One who wishes to become a master of the science of demonstration should first obtain a good grasp of probabilities. Whereas the principles of demonstrative logic are necessary; those of dialectic are probable." (John of Salisbury, "Metalogicon", 1159)

"[…] he who wishes to attain to human perfection, must therefore first study Logic, next the various branches of Mathematics in their proper order, then Physics, and lastly Metaphysics." (Moses Maimonides, "The Guide for the Perplexed", 1190)

"Men are further beholding […] generally to chance, or anything else, than to logic, for the invention or arts and sciences." (Francis Bacon, "The Advancement of Learning", 1605)

"XI. As the present sciences are useless for the discovery of effects, so the present system of logic is useless for the discovery of the sciences. XII. The present system of logic rather assists in confirming and rendering inveterate the errors founded on vulgar notions than in searching after truth, and is therefore more hurtful than useful."  (Sir Francis Bacon, "Novum Organum", 1620)

"In logic, they teach that contraries laid together more evidently appear: it follows, then, that all controversy being permitted, falsehood will appear more false, and truth the more true; which must needs conduce much to the general confirmation of an implicit truth." (John Milton, "True Religion, Heresy, Schism, Toleration, and what best means may be used against the Growth of Popery", 1673)

"Nothing universal can be rationally affirmed on any moral or any political subject. Pure metaphysical abstraction does not belong to these matters. The lines of morality are not like the ideal lines of mathematics. They are broad and deep as well as long. They admit of exceptions; they demand modifications. These exceptions and modifications are not made by the process of logic, but by the rules of prudence. Prudence is not only the first in rank of the virtues political and moral, but she is the director, regulator, the standard of them all. Metaphysics cannot live without definition; but prudence is cautious how she defines." (Edmund Burke, "Appeal from the New to the Old Whigs", 1791)

"The advantages which mathematics derives from the peculiar nature of those relations about which it is conversant, from its simple and definite phraseology, and from the severe logic so admirably displayed in the concatenation of its innumerable theorems, are indeed immense, and well entitled to separate and ample illustration." (Dugald Stewart, "Philosophy of the Human Mind", 1792)