Summa Theologica, by St. Thomas Aquinas, , at sacred-texts.com
Since, as Damascene states (De Fide Orth. ii, 12), man is said to be made in God's image, in so far as the image implies "an intelligent being endowed with free-will and self-movement": now that we have treated of the exemplar, i.e. God, and of those things which came forth from the power of God in accordance with His will; it remains for us to treat of His image, i.e. man, inasmuch as he too is the principle of his actions, as having free-will and control of his actions.