the affirmation of their contradictories; such premises, then, hold that standing.
As for propositions that are not of this type but are composed of ordinary, experiential judgments—such as those treated in physics, medicine, grammar, and the like—these are often shown to be mistaken, and the intellect does not decisively rule out their refutation except under certain conditions. Matters that rest on habit are liable to exception; and once a universal proposition admits of exception, it is, in their own system, no longer material for demonstration but merely for dialectic and rhetoric.
Should it be objected, “Yet you yourselves are certain of what a speaker intends in most situations: you are sure of the intent of the Messenger, of the jurists, of the physicians and grammarians—and all linguistic knowledge is founded on customary propositions,” we reply: certainty about the intent of a particular person is not a universal proposition at all; it is knowledge of the intention of that specific individual. Although our familiarity with his language and customary usage helps us discern his meaning, we still need a knowledge peculiar to him by which we know that he employed that usage expressly to convey his intent to us. This, then, lies outside the present discussion, and we do not establish it solely by means of the demonstration they describe.
They fault the discipline of definition for making the manifest known through what is obscure; the fault is even graver in the discipline of demonstration. Demonstration is sought only to clarify, disclose, and illuminate the thing signified; if the proof itself is more obscure, it ends up making the evident known through what is hidden. As for definition, the correct view is that its aim is to distinguish the thing from what resembles it, not to lay bare its very quiddity; and since the goal is distinction, the distinguishing mark may sometimes be more obscure and sometimes more evident.
Hidden premises may nonetheless benefit certain people, especially in debate. Because clarity and obscurity are relative, some people profit from a subtle proof or a subtle definition. Many, when a self-evident point is put before them, make light of it and will not concede until they are given a proof whose very occurrence necessitates the point; then they accept it—just as they yield when a definition is supplied that singles the matter out. This is usually the case with a stubborn disputant, or with one whose soul has grown accustomed to knowing only what it has wrestled with, pondered over, and pursued from premise to premise. Habit becomes a second nature, and many who are addicted to disputation and inquiry have turned that habit into something like nature itself: they neither recognize, nor accept, nor submit.