α) St. Augustine embodies all these considerations in the following passage: “It is certain that we keep the commandments when we will; but because the will is prepared by the Lord, we must ask of Him that we may will so much as is sufficient to make us act in willing. It is certain that we will whenever we like, but it is He who makes us will what is good, of whom it is said (Prov. VIII, 35): ‘The will is prepared by the Lord,’ and of whom it is said (Ps. XXXVI, 32): ‘The steps of a [good] man are ordered by the Lord, and his way doth He will,’ and of whom it is said (Phil. II, 13): ‘It is God who worketh in you, even to will.’ It is certain that we act whenever we set to work; but it is He who causes us to act, by giving thoroughly efficacious powers to our will, who has said (Ezech. XXXVI, 27): ‘I will cause you to walk in my commandments, and to keep my judgments, and do them.’ When He says: ‘I will cause you ... to do them,’ what else does He say in fact than (Ezech. XI, 19): ‘I will take away the stony heart out of their flesh,’ from which used to rise your inability to act, and (Ezech. XXXVI, 26): ‘I will give you a heart of flesh,’ in order that you may act.”(97)
β) The manner in which grace and free-will coöperate is a profound philosophical and theological problem. A salutary act derives its supernatural character from God, its vitality from the human will. How do these two factors conjointly produce one and the same act? The unity of the act would be destroyed if God and the free-will of man in each case performed, either two separate acts, or each half of the same act. To preserve the unity of a supernatural act two conditions are required: (1) the divine power of grace must be transformed into the vital strength of the will and (2) the created will, which by its own power can perform at most a naturally good act, must be equipped with the supernatural power of grace. These conditions are met (a) by the supernatural elevation of the will (_elevatio externa_), and (b) by the supernatural concurrence of God (_concursus supernaturalis ad actum secundum_). The supernatural elevation of the will is accomplished in this wise: God, by employing the illuminating and strengthening grace, works on the _potentia obœdientialis_, and thus raises the will above its purely natural powers and constitutes it a supernatural faculty _in actu primo_ for the free performance of a salutary act. The divine concursus supervenes to enable the will to perform the _actus secundus_ or salutary act proper. This special divine concurrence, in contradistinction to the natural concursus whereby God supports the created universe,(98) is a strictly supernatural and gratuitous gift. Consequently, God and the human will jointly perform one and the same salutary act—God as the principal, the will as the instrumental cause.(99)