SEGNO. — In the broad sense, s. (Gr. σημεῖον) is anything (reality, image, object) that manifests something else: «quodcumque notum in quo aliquid cognoscatur» (s. Tommaso, De verit., q. 9, a. 4 ad 4; cf. Sum. Theol., 3ª, q. 60, a. 4: «signum est per quod aliquis devenit in cognitionem alterius»): thus breathing and the beating of the heart are s. of life. In a stricter meaning, s. is that which exists, or is taken up, solely as the expression of something else: in this sense the flag is s. of the homeland, the word s. of the idea.
Lalande (Vocab. techn. et crit. de la philos., 5ª ed., Paris 1947, pp. 970-71) proposes a threefold distinction: s. is: 1) an actual perception that permits, with greater or lesser certainty, an affirmation concerning something else: in this sense it may be said that the scope of s. is extremely broad, owing to the multiplicity of relations linking the various aspects of a reality and the different realities among themselves; 2) an expressive act or gesture indicating a command or an action to be performed; 3) an object (figure, image) serving exclusively as a symbol of something else and recalling its representation or idea, e.g., the symbols of the chemical elements: this is s. in the strict sense already defined. The further distinction between natural and conventional s. is based on the nature of the relation existing between the s. and the thing signified: “natural” is the s. that entails a real relation (of cause, effect, or likeness: where the latter relation intervenes, one sometimes speaks of a “formal” s.; in this sense the concept as the representation or intentional form of the object is called a “formal” s.: cf. s. Tommaso, De verit., loc. cit.), e.g., a footprint indicating the passage and direction of an animal, or the various physiognomic s. as expressions of feelings and psychic qualities (a semantics already attempted empirically by Aristotle: cf. Physiognom., especially chap. 3). If, on the other hand, the relation between s. and the thing signified is established by pure convention, the s. is “conventional,” e.g., the s. of musical language. The division is by extremes and admits intermediate degrees, since in some objects there may be a certain aptitude to be taken up as s. of a particular thing, e.g., the scales as s. of justice or, in the Christian religion, Baptism as s. of inward spiritual purification. As regards human language,
it may be said that it comprises all the gradations of the semantic relationship, from the cry, an immediate expression of a feeling, to onomatopoeic or structurally and phonetically expressive words, and finally to the purely conventional and arbitrary words of modern scientific languages.
The significance of s. is important in human knowledge, not only practical, where its function is well known, but also theoretical. Aristotle had already noted its philosophical importance, insofar as “it is proper to philosophy sometimes to be able to discover something necessary where certain s. present themselves” (Physiognom., chap. 2, 807 a 9-10). In reality, understanding s. in its broadest meaning, it is through sensible s. (the exterior characteristics of the world and of its activity, which reveal contingency) that, according to s. Tommaso, it is possible to arrive at the affirmation of God. It is likewise s. Tommaso who observes: «quod signa proprie dantur hominibus quorum est per nota ad ignota pervenire» (Sum. Theol., 3ª, q. 60, a. 2, c.; cf. De verit., loc. cit.). It is therefore perfectly consonant with the character of human knowledge and activity that supernatural reality and its communication to man should be expressed in the Christian religion through sensible s. (Sacrifice, Sacraments).