SINDERESIS. - A medieval term equivalent to the modern "moral sense" understood as "the spiritual quality by which we know the first principles of the practical order, which regulate all practical knowledge" (Y. Simon, *Critique de la connaissance morale*, Paris 1934, p. 56). Synderesis is thus distinct—though it is its ultimate root—from "moral conscience" (*conscientia moralis*).
The term appears to derive from συντήρησις (*syntērēsis*), meaning "to watch attentively, observe," and was used by the Stoics; however, it is uncertain whether they saw a difference in meaning between συντήρησις and συντήρησις (cf. M. Waldmann, *Syntheses oder Synedesis? Ein Beitrag zur Lehre vom Gewissen*, in *Theol. Quartalschr.*, 109 [1938], pp. 332 ff.). It entered Scholasticism through a text of St. Jerome, who, above the three faculties of the soul (rational, irascible, concupiscible), posits a fourth: "which the Greeks call συντήρησις," *quae scintilla conscientiae in Cain quoque pectore, postquam eiectus est de paradiso, non extinguitur, et qua victi voluptatibus vel furore, ipsaque interdum rationis decepti similitudine, nos peccare sentimus... Et tamen hanc quoque conscientiam... cernimus praecipitari apud quosdam et suum locum ammittere...* (*Comm. in Ezech.*, I, cap. 1: *PL* 25, 22). Among the Scholastics, the term is already found in Magister Udo, in his commentary (c. 1160) on Peter Lombard, who was the first to examine St. Jerome’s text (*Libri IV Sententiarum*, II, dist. 39, at the end). Studied and interpreted in various ways in the 12th and 13th centuries, synderesis received its two fundamental interpretations from St. Bonaventure and St. Thomas.
St. Bonaventure (*In II Sent.*, dist. 39, aa. 1–2) regards synderesis as "a power inasmuch as it is naturally inclined toward the good and tends toward it," i.e., the same natural will insofar as it is inclined to moral good; it is thus distinct from conscience, which is a natural judge directing the intellect in knowing. St. Thomas, however (*In II Sent.*, dist. 24, q. 2, aa. 3–4; dist. 39, q. 3, a. 1; *De verit.*, q. 16; *Sum. Theol.*, 1a, q. 79, aa. 12–13; 1a–2ae, q. 94, a. 1; 2a–2ae, q. 47, a. 6), places synderesis on the level of practical knowledge, as a natural intellectual habit of the first principles directing human activity.