Summa Theologiae Moralis, vol. I: De Principiis

220. Object of the Treatise. — After the consideration of human acts in themselves and of their proximate, immediate, and intrinsic rule, which is conscience, there logically follows the treatise on the remote and extrinsic rule, as the extrinsic and regulative principle of conscience itself, according to which acts must be directed to the end, namely on law, which conscience must apply in each of our actions, and thus conform and subordinate itself to it.

In this treatise we shall speak not of individual laws considered specifically, but of law itself and of its highest species considered in general, namely

1° of law in general: a) of law in itself, b) of its division, c) of its effects; — then specifically:

2° of eternal law, 3° of natural law, 4° of human or civil law, 5° of divine law, 6° of ecclesiastical law; and for each we shall speak of a) its essence, b) its power, c) its mutability.

These six parts are reduced to three sections: the first two regard law rather according to its psychological being, — the next two regard law as the rule of morally good acts, according to its moral being, — the last two regard law also as the rule of supernatural acts, according to its supernatural being¹.