§ 1. Reasoning and syllogism

48. Reasoning. -- The aim of all intellectual processes is the knowledge of truth.

Certain truths are known immediately; others, mediately, by means of those known immediately. The former, as generating the latter, are called principles; the latter are consequences, or conclusions. To proceed from principles to conclusions is to reason.

A conclusion is a proposition, and, as such asserts a predicate of a subject. When the predicate manifestly belongs to the subject, the proposition is evident.^{1} This evidence is immediate when the objective connection between the predicate and the subject of a judgment is immediately apparent to the intelligence; also immediate is the corresponding certitude. But in most cases the evidence of the judgment is brought to light only by the employment of one or more intermediaries, or middle terms -- common terms of comparison between subject and predicate. In such cases the evidence is mediate, or by reasoning, as is the corresponding certitude. The kind of evidence is the evidence proper to conclusions.

The necessity of this discursive proceeding arises from the disproportion between the complexity of intelligible things and the inadequacy of the intelligence which is called upon to know them.^{2} The power of reasoning urges a perfection which the metaphysicians call mixed, i. e., marked by imperfection.

It is a perfection to be able to reason, i. e., to reach the knowledge of truths which otherwise would remain unknown.

It is an imperfection to be obliged to reason, i. e., to reach the truth only by winding and difficult paths.

{1} See Criteriology.

{2} "The discourse of reason always begins in the understanding and ends in the thing understood; for we reason by proceeding from certain understood things; and the discourse of reason is complete when we arrive at the understanding of what was previously unknown. Our reasoning, therefore, proceeds from some precedent understanding." Summa Theol., 2a 2ae, q. 8, a. I, ad 2.