In scholastic philosophy, synderesis is the inborn moral consciousness that distinguishes between good and evil. The word is really synteresis, but synderesis is the commoner form.
Diogenes Laertius in his account of the Stoics uses the phrase ???rripeiv eavro??? to describe the instinct for self-preservation, the inward harmony of Chrysippus, the recognition of which is ???avveidriffis???. The term synderesis, however, is not found till Jerome, who in dealing with Ezekiel i. 4-15, says the fourth of the "living creatures" of the vision is what the Greeks call (???rvvrriprjais???, i.e. scintilla conscientiae the "spark of conscience". Hence apparently synderesis and conscience (???vveiSyais???) are equivalent.
The schoolmen, however, made a distinction between the two, conscience being the practical envisaging of good and evil actions; synderesis being, so to speak, the tendency toward good in thought and action. The exact relation between the two was, however, a matter of controversy, Aquinas and Duns Scotus holding that both are practical reason, while Bonaventure narrows synderesis to the volitional tendency to good actions.