On the Freedom of the Will
Having now repelled these objections by the answer which we have given, let us hasten on to the discussion of the subject itself, in which it is said, “It is not of him that wills, nor of him that runs, but of God that shows mercy.” In the book of Psalms— in the Songs of Degrees, which are ascribed to Solomon— the following statement occurs: “Except the Lord build the house, they labour in vain that build it; except the Lord keep the city, the watchman wakes but in vain.” By which words he does not indeed indicate that we should cease from building or watching over the safe keeping of that city which is within us; but what he points out is this, that whatever is built without God, and whatever is guarded without him, is built in vain, and guarded to no purpose. For in all things that are well built and well protected, the Lord is held to be the cause either of the building or of its protection. As if, e.g., we were to behold some magnificent structure and mass of splendid building reared with beauteous architectural skill, would we not justly and deservedly say that such was built not by human power, but by divine help and might? And yet from such a statement it will not be meant that the labour and industry of human effort were inactive, and effected nothing at all. Or again, if we were to see some city surrounded by a severe blockade of the enemy, in which threatening engines were brought against the walls, and the place hard pressed by a vallum, and weapons, and fire, and all the instruments of war, by which destruction is prepared, would we not rightly and deservedly say, if the enemy were repelled and put to flight, that the deliverance had been wrought for the liberated city by God? And yet we would not mean, by so speaking, that either the vigilance of the sentinels, or the alertness of the young men, or the protection of the guards, had been wanting. And the apostle also must be understood in a similar manner, because the human will alone is not sufficient to obtain salvation; nor is any mortal running able to win the heavenly (rewards), and to obtain the prize of our high calling of God in Christ Jesus, unless this very good will of ours, and ready purpose, and whatever that diligence within us may be, be aided or furnished with divine help. And therefore most logically did the apostle say, that “it is not of him that wills, nor of him that runs, but of God that shows mercy;” in the same manner as if we were to say of agriculture what is actually written: “I planted, Apollos watered; but God gave the increase. So then neither is he that plants anything, neither he that waters; but God that gives the increase.” As, therefore, when a field has brought good and rich crops to perfect maturity, no one would piously and logically assert that the husbandman had made those fruits, but would acknowledge that they had been produced by God; so also is our own perfection brought about, not indeed by our remaining inactive and idle, (but by some activity on our part): and yet the consummation of it will not be ascribed to us, but to God, who is the first and chief cause of the work. So, when a ship has overcome the dangers of the sea, although the result be accomplished by great labour on the part of the sailors, and by the aid of all the art of navigation, and by the zeal and carefulness of the pilot, and by the favouring influence of the breezes, and the careful observation of the signs of the stars, no one in his sound senses would ascribe the safety of the vessel, when, after being tossed by the waves, and wearied by the billows, it has at last reached the harbour in safety, to anything else than to the mercy of God. Not even the sailors or pilot venture to say, “I have saved the ship,” but they refer all to the mercy of God; not that they feel that they have contributed no skill or labour to save the ship, but because they know that while they contributed the labour, the safety of the vessel was ensured by God. So also in the race of our life we ourselves must expend labour, and bring diligence and zeal to bear; but it is from God that salvation is to be hoped for as the fruit of our labour. Otherwise, if God demand none of our labour, His commandments will appear to be superfluous. In vain, also, does Paul blame some for having fallen from the truth, and praise others for abiding in the faith; and to no purpose does he deliver certain precepts and institutions to the Churches: in vain, also, do we ourselves either desire or run after what is good. But it is certain that these things are not done in vain; and it is certain that neither do the apostles give instructions in vain, nor the Lord enact laws without a reason. It follows, therefore, that we declare it to be in vain, rather, for the heretics to speak evil of these good declarations.
19. After this there followed this point, that “to will and to do are of God.” Our opponents maintain that if to will be of God, and if to do be of Him, or if, whether we act or desire well or ill, it be of God, then in that case we are not possessed of free-will. Now to this we have to answer, that the words of the apostle do not say that to will evil is of God, or that to will good is of Him; nor that to do good or evil is of God; but his statement is a general one, that to will and to do are of God. For as we have from God this very quality, that we are men, that we breathe, that we move; so also we have from God (the faculty) by which we will, as if we were to say that our power of motion is from God, or that the performing of these duties by the individual members, and their movements, are from God. From which, certainly, I do not understand this, that because the hand moves, e.g., to punish unjustly, or to commit an act of theft, the act is of God, but only that the power of motion is from God; while it is our duty to turn those movements, the power of executing which we have from God, either to purposes of good or evil. And so what the apostle says is, that we receive indeed the power of volition, but that we misuse the will either to good or evil desires. In a similar way, also, we must judge of results.
Source: De Principiis (New Advent)