/summa/FS/FSVI/FS_Q90:
We have now to consider the extrinsic principles of acts. Now the extrinsic principle inclining to evil is the devil, of whose temptations we have spoken in the FP, Q[114]. But the extrinsic principle moving to good is God, Who both instructs us by means of His Law, and assists us by His Grace: wherefore in the first place we must speak of law; in the second place, of grace.
Concerning law, we must consider: (1) Law itself in general; (2) its parts. Concerning law in general three points offer themselves for our consideration: (1) Its essence; (2) The different kinds of law; (3) The effects of law.
Under the first head there are four points of inquiry:
(1) Whether law is something pertaining to reason?
(2) Concerning the end of law;
(3) Its cause;
(4) The promulgation of law.