Didascalicon, Book I (chapters 2-13): the origin of the arts and the division of philosophy
Chapter XII. On the rise of logic
Now that we have shown the rise of the theoretical, the practical and the mechanical, it remains to investigate the origin of logic also; which I number last for this reason, that it was discovered last. The others had been discovered earlier; but it was necessary that logic also be discovered, since no one can discourse suitably about things unless he has first learned the method of speaking correctly. For as Boethius says: When the ancients first applied their labors to investigating the natures of things and the qualities of morals, it was inevitable that they should often be deceived, because they had no discernment between words and concepts; as happened in many matters to Epicurus, who thinks the world consists of atoms, and measures the honorable by pleasure.
It is clear that this happened to him and to others for this reason, that whatever they had grasped by reasoning in disputation, they supposed to hold also in the things themselves. But this is a great error; for things are not in reasonings as they are in numbers. In numbers, whatever turns out on the fingers of one who computes rightly must without doubt turn out in the things themselves; so that if by calculation a hundred results, there must be a hundred things subject to that number. But this is not equally preserved in disputation. For whatever the course of speech has found is not held fixed in nature. Hence those who, having cast aside the science of disputing, inquire into the nature of things must needs be deceived. For unless one has first come to the knowledge of what reasoning keeps the true path of disputing, what recognizes the probable, and which is trustworthy and which may be suspect, the uncorrupted truth of things cannot be found by reasoning. Since, therefore, the ancients, often falling into many errors, gathered in disputation certain false and self-contradictory conclusions, and it seemed impossible that, a contrary conclusion having been drawn on the same matter, both should be true which the reasoning had concluded against itself, and it was uncertain which reasoning should be believed, it seemed good first to consider the true and complete nature of disputation itself; this being known, it could also be understood whether what was found by disputation had been truly comprehended. From this, then, arose logic, the skill of the discipline which prepares the modes of disputing and the ways of discerning reasonings themselves, so that it may be recognized which reasoning is at one time true, at another false; which is always false, which never false. In time it is last, but in order first; for to those beginning philosophy it is to be read first, because in it is taught the nature of words and concepts, without which no treatise of philosophy can be reasonably unfolded.
Logic is so called from the Greek word logos, which has a twofold interpretation. For it means speech or reason; and hence logic can be called the science of speech or the science of reason. Rational logic, which is called discerning, contains dialectic and rhetoric. Sermocinal logic is a genus to grammar, dialectic and rhetoric, and contains under it the art of discourse. And this is the sermocinal logic which we count as fourth after the theoretical, practical and mechanical. Nor is it to be thought that logic, that is the sermocinal, is so called because before its discovery there were no speeches, and as though men had not had mutual converse before. There were before both common speech and letters; but the method of speech and letters had not yet been brought into use, and no precepts of speaking or disputing rightly had yet been given. For all the sciences were in use before they were in art. But men, considering afterwards that use could be converted into art, and that what had been vague and licentious before could be restrained by fixed rules and precepts, began, as has been said, to reduce custom, which had arisen partly by chance, partly by nature, to art: correcting what was faulty in usage, supplying what it lacked, cutting off what was superfluous, and for the rest prescribing fixed rules and precepts for each.
Such was the origin of all the arts; running through them one by one, we find it true. Before there was grammar, men wrote and spoke; before there was dialectic, they discerned the true from the false by reasoning; before there was rhetoric, they handled civil rights; before there was arithmetic, they had the practice of counting; before there was music, they sang; before there was geometry, they measured fields; before there was astronomy, they took the distinctions of times from the courses of the stars. But the arts came, which, though they took their beginning from use, are nevertheless better than use. This would be the place to set out who were the inventors of the several arts, when they arose, or where, or how the discipline took its beginning through them; but I wish first to distinguish each from the others by a certain division of the philosophers.