SIGN. — In a broad sense, a sign (Gk. σημαῖον) is any thing (reality, image, object) that manifests another: “quodcumque notum in quo aliquid cognoscitur” (St Thomas Aquinas, 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 breath and the beating of the heart are signs of life. In a stricter sense, it is that which exists or is assumed solely as an expression of another thing: in this sense the flag is a sign of the fatherland, the word a sign of the idea.
Lalande (Vocab. techn. et crit. de la philos., 5th ed., Paris 1947, pp. 970–71) proposes a threefold distinction: a sign is: 1) an actual perception that, with greater or lesser certainty, enables an assertion about another thing; in this sense the extension of the term is very wide, owing to the multiplicity of relations linking the various aspects of a reality and the different realities among themselves; 2) an act or expressive gesture commanding or directing an action to be performed; 3) an object (figure, image) serving exclusively as a symbol of another thing and recalling its representation or idea, e.g., the symbols of chemical elements: this is the sign in the strict sense already defined. The further distinction of the sign into natural and conventional rests on the nature of the relation between the sign and the thing signified: “natural” is the sign that involves a real relation (of cause, effect, or resemblance; when the latter relation is involved, one sometimes speaks of a “formal” sign, and in this sense the concept is called a “formal” sign as the representation or intentional form of the object: cf. St Thomas Aquinas, De verit., loc. cit.), e.g., the footprint indicating the passage of an animal and its direction, or the various physiognomic signs as expressions of feelings and psychic qualities (an empirical attempt at semantics already made by Aristotle: cf. Physiognomonica, especially ch. 3). If, however, the relation between sign and thing signified is established purely by convention, the sign is “conventional,” e.g., the signs of musical notation. The division is by extremes and admits of intermediate degrees, since in some objects there may be a certain aptitude to be taken as signs of a given thing, e.g., the scales as a sign of justice or, in Christian religion, Baptism as a sign of internal spiritual purification. With regard to human language, it may be said to comprise all gradations of the semantic relation, from the cry, an immediate expression of feeling, to onomatopoeic or structurally and phonetically expressive words, up to the purely conventional and arbitrary terms of modern scientific languages.
The significance of the sign is important in human knowledge, not only practical, where its function is well known, but also theoretical. Already Aristotle noted its philosophical importance in that “it is proper to philosophy to be able sometimes to discover something necessary where certain signs present themselves” (Physiognomonica, ch. 2, 807 a 9–10). Indeed, taking the sign in its broadest sense, it is through sensible signs (the outward features of the world and its activity, revealing contingency) that, according to St Thomas Aquinas, one can arrive at the affirmation of God. The same Aquinas 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 nature of human knowledge and activity that the supernatural reality and its communication to man should be expressed in Christian religion through sensible signs (Sacrifice, Sacraments).