Summa Theologiae Moralis, vol. I: De Principiis

198. Object of the Treatise. — After the consideration of the end and of human acts in themselves (according to their material and formal element), we must treat 1) of the rules of acts according to which they must be directed (exemplary cause), and 2) of their principles (efficient cause). The principles are the powers of man, which philosophy treats, and habits whether good or bad, which are discussed in the treatise on habits and virtues. The rules, however, are two: a remote one, objective and extrinsic, which is law, and a proximate one, subjective and intrinsic, or immediate, which is conscience. Hence after the treatise on human acts there logically follows the treatise on their proximate rule, conscience. For just as an archer must know the rules according to which he ought rightly to aim at the target, so man must know the rules according to which his acts may be ordered to the last end.

Moreover, general morals considers conscience only in general, what it must be in order to be the rule of our acts and to have for this purpose the required endowments; but in no way does it consider the individual particular states or all the species of conscience, nor the ways of prudently forming conscience or of sinning against it: for all these things already pertain to special morals, and indeed to the treatise on the cardinal virtue of prudence, whose act is conscience right and well formed. In the present work, therefore, we shall discuss 3 questions¹.

1. on moral conscience according to itself (physical or psychological being);

2. on conscience as it is the rule of moral acts (moral being) or on the qualities required for moral conscience, namely a) on its truth and rectitude on the part of the object; b) on its certitude on the part of the subject;

3. on conscience as it is the rule of supernatural acts (supernatural being).