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Synderesis, or more correctly synteresis, is a term used by the Scholastic theologians to signify the habitual knowledge of the universal practical principles of moral action. The reasoning process in the field of speculative science presupposes certain fundamental axioms on which all science rests. Such are the principle of contradiction, "a thing cannot be and not be at the same time," and self-evident truths like "the whole is greater than its part". These are the first principles of the speculative intellect. In the field of moral conduct there are similar first principles of action, such as: "evil must be avoided, good done"; "Do not to others what you would not wish to be done to yourself"; "Parents should be honoured"; "We should live temperately and act justly". Such as these are self-evident truths in the field of moral conduct which any sane person will admit if he understands them. According to the Scholastics, the readiness with which such moral truths are apprehended by the practical intellect is due to the natural habit impressed on the cognitive faculty which they call synderesis. While conscience is a dictate of the practical reason deciding that any particular action is right or wrong, synderesis is a dictate of the same practical reason which has for its object the first general principles of moral action.
ST. THOMAS, Summa, I, Q. lxxix, a. 12 (Parma, 1852); PATUZZI, De ratione humana in MIGNE, Theologiae Cursus completus, XI (Paris, 1841).