REASONING. — From ratio (v. RAGIONE), it is the act by which thought, in virtue of given propositions, facts, and judgments, proceeds to a new affirmation. R. is therefore, in its broadest meaning, synonymous with inference, and denotes the “discourse” or movement of reason from knowledge in act (premises, προτάσεις, τὰ προτεινόμενα) to further judgments (conclusions), having with the former a relation of causal derivation. The logical relation of derivation (“consequent” in scholastic terminology) is essential; hence every process of thought that takes place through chance discovery or simple succession in time cannot be called r.
In traditional logic, r. is, concept (v.) judgment (v.), the third fundamental activity of reason. St. Thomas sees in it the distinctive character of human understanding, essentially analytical and progressive in time: “homines ad intelligibile veritatem cognoscendam perveniunt procedendo de uno ad aliud ... et ideo rationales dicuntur” (Sum. Theol., 1°, q. 79, a. 8, c.; cf. De verit., q. 15, a. 1). Nevertheless, the knowing principle that understands (intellectus) and reasons (ratio) is identical; the difference and multiplicity are merely functional: whereas the intellect denotes “pure apprehension of truth,” it is proper to reason “discurrere ab uno in aliud, ut per id quod est notum deveniat in cognitionem ignoti” (In anal. post., I, lect. 1, n. 4; cf. Sum. Theol., loc. cit., and q. 85, a. 4). R. is therefore, in relation to intellection, as “acquisition” to “attainment,” “becoming” to “being,” “motion” to “rest.” In analyzing this relation, St. Thomas observes that, just as movement is impossible without a fixed point of departure, so r. too presupposes certain truths or principles of immediate intuition (“intellectus principiorum”) from which it begins and to which it returns in the resolving and demonstrative process of reaching conclusions. “Ratio” therefore finds in “intellectus” both its principle and its term: “intellectus invenitur rationis principium quantum ad viam inveniendi, terminus vero quantum ad viam iudicandi” (De verit., q. 15 a. 1).
As inference, r. is an essentially active function of thought: not pure intuition of a datum, but a process of inquiry (“a certis ad incertorum indagationem nitens cogitatio”; St. Augustine, De immort. anim., cap. 1: PL 32, 1021 A). It is clear, however, that the intuitive moment is not absent from r.; indeed, it assumes there more complex forms than in the pure concept. R. takes place, in fact, through a precise noetic act in which a certain proposition B is seen to depend upon and derive from a preceding proposition or series of propositions A. R. is therefore intuition-affirmation of judgments (relations between concepts) and of causal connections between judgments: it consequently involves multiple intuitive moments that make r. almost a synthesis of the other mental acts, and through which the indicated distinction between “intellectus” and “ratio” is further attenuated.
R. as inference does not necessarily imply the discovery of new truths, as in the case of a proposition already known, derived (proved) from a principle that justifies or confirms it (e.g.: theft is an evil because it is incompatible with social life, and society is a good for man). The novelty of r. (which in reality is always present, otherwise it would not be possible to speak of genuine inference or rational process) often consists precisely in proof: B was known as problematic; now it is seen in its connection with A, which is certain: r. here establishes the relation of derivation of B from A and thus arrives at a new certainty and evidence of B (even if it may already have been known by another route).
With regard to the various forms of inference (for which V. SILLOGISMO), the general division of r. deduction (v.) induction (v.) should be noted here. Whereas deduction verifies the type of formally and logically necessary inference (to which the rule “ex vero non sequitur nisi verum” principally refers: cf. Aristotle, Anal. pr., II, cap. 2, 53 b 7), in induction, on the other hand, inference from particular truths to universal principles is based less on the logical structure of r. than on the intellectual intuition of the relation of particular facts to the ontological determination of universal nature. But it would be an arbitrary restriction to understand r. solely as formal inference (cf. A. Lalande, op. cit. in bibl., p. 868), since induction too verifies an actual process and dialectic of thought. Equally unjustified, on the other hand, is the contrary assertion (cf. J. Stuart Mill, A system of logic, II, cap. 3), which sees only in induction a true inference, in the sense of an enlargement and extension of knowledge.
R. as a process in time (and therefore open to a genuine “progress” of knowledge; for Bosanquet, the denial of the temporal duration of inference “is the first and fatal error” [op. cit. in bibl., p. 4]) and as dialectical development is an essential form of rational thought. It is determined, on the one hand, by the fragmentary and analytical character of human knowing and, on the other, by the indefinite relations that bind the object and contents of thought into unity. Movement of knowledge toward unity, analysis toward synthesis, r. is the logical expression of the ontological organicity of reality. It is therefore the perfection and power of thought which, while extending the domain of truth in breadth, simultaneously clarifies it in depth, determining its manifold internal relations of structure, necessity, and cause.