Didascalicon, Book II: the sciences, their parts and their discipline
Chapter XXXI. On the method of discoursing
The method of discoursing has integral parts, invention and judgment; and divisive parts, demonstration, the probable, and the sophistical. Demonstration is in necessary arguments, and pertains to philosophers. The probable pertains to dialecticians and rhetoricians. The sophistical to sophists and quibblers. The probable is divided into dialectic and rhetoric, each of which has as integral parts invention and judgment. For since they integrally constitute the genus itself, that is the art of discoursing, it is necessary that in the composition of all its species they be found together. Invention is that which teaches how to find arguments and construct argumentations. Judgment is the science of judging, which teaches how to judge of both. It may be asked whether invention and judgment are contained under philosophy. For they seem to be contained neither under the theoretical, nor the practical, nor the mechanical, nor the logical (of which it would seem more likely). Under logic they are not contained, because neither through grammar nor through the art of discoursing. Through the art of discoursing they are not contained, since they constitute it integrally; and nothing can be at once an integral and a divisive part of the same genus. And so philosophy seems not to contain all knowledge. But it must be known that knowledge is usually taken in two ways: for some one of the disciplines, as when I say dialectic is a science, that is an art or discipline; and for any cognition whatever, as when I say that he has knowledge who knows something. For example: if I know dialectic, I have knowledge; and if I know how to write shorthand, I have knowledge; and if I know that Socrates is the son of Sophroniscus, I have knowledge. And universally, everyone who knows anything can be said to have knowledge. Yet it is one thing when I say, dialectic is a science, that is an art or discipline; and another when I say, to know that Socrates is the son of Sophroniscus is knowledge, that is cognition. Of all knowledge which is an art or discipline it is true to say that it is a divisive part of philosophy; but it cannot be said universally that all knowledge, which is cognition, is a divisive part of philosophy. Yet every knowledge, whether discipline or any cognition whatever, is a part of philosophy, either divisive or integral. Discipline, however, is knowledge that has an absolute end in which the aim of the art is perfectly unfolded, which does not belong to the knowledge of inventing or of judging; for neither is complete by itself, and therefore they cannot be called disciplines, but parts of a discipline, namely of the art of discoursing. Again it is asked whether invention and judgment are the same parts of dialectic and rhetoric; which seems unsuitable, that two opposite genera should be constituted of altogether the same parts. It can be said, then, that these two terms are equivocal with respect to the parts of dialectic and rhetoric, or (what is perhaps better) let us say that invention and judgment are properly parts of the art of discoursing, and are univocal under these words. In the lower members of this genus, however, they differ by certain properties, which by these words are not distinguished; because they are signified by them not according to the fact that they compose species, but according to the fact that they are parts of the genus. Grammar is the science of speaking without fault. Dialectic is acute disputation, distinguishing the true from the false. Rhetoric is the discipline suited to persuading.