Cursus Philosophicus Thomisticus, Prima Pars Artis Logicae

Before treating the principal perfect expression, which is the proposition, we must treat the method of knowing. For although many things pertaining to the method of knowing are found in the third operation of the intellect, namely argumentation, other such things are found in the second operation, and some pertain even to imperfect expression. Therefore the method of knowing in general is not restricted to perfect or imperfect expression; accordingly, before treating perfect expression specifically, it is fitting to treat the method of knowing in general, which can be found in both perfect and imperfect expression.

Therefore the method of knowing in general is defined as “an expression that makes something unknown manifest.” Note that making manifest is one thing and signifying another. For although a sign seems to make the thing signified manifest to a power, the making manifest of which we now speak is different from signifying absolutely. “Manifest” is taken in two ways: first, as opposed to obscure; second, as opposed to unknown and not presented to a cognitive power. In the first sense, manifestation occurs through something better known and clearer, by which that obscurity is removed. Thus, to make something unknown manifest in our intellect pertains to the method of knowing, provided that this is done by means of an expression. For manifestation by a formal rationale that makes something manifest in a simple way—for example, the authority of a speaker making credible things manifest—is not a method of knowing, because it is not an expression or complex explanation, that is, one ordered to knowledge and having an inferential mode. In the second sense, manifestation occurs through some means or instrument that presents to us an object that was not presented; in this way, making manifest pertains to a sign or representation, speaking generally.

When, then, something unknown or obscure needs to be explained by an expression that explains it and removes that obscurity, such an expression is called a method of knowing, because every manifestation is called knowledge or is ordered to knowledge. Two things can be made manifest to the intellect by an expression: something incomplex or some complex truth. An incomplex thing, such as man, heaven, or earth, is explained by definition with respect to the obscurity of its quiddity, or by division with respect to the confusion of the parts or of the multitude contained in some thing. A complex truth, if obscure or doubtful, is made manifest by proof. Proof is made by reasoning and consequence, and thus is argumentation.

Hence the method of knowing is adequately divided, on the part of the things that can be made manifest, into definition, division, and argumentation. For this reason, definition and division are sometimes said to pertain to the first operation, or simple apprehension, by reason of their object, which they make manifest: something incomplex.