21 The soul is self-governed: and though the devil can suggest, he has not the power to compel against the will. He pictures to you the thought of fornication: if you will, you accept it; if you will not, you reject. For if you were a fornicator by necessity, then for what cause did God prepare hell? If you were a doer of righteousness by nature and not by will, wherefore did God prepare crowns of ineffable glory? The sheep is gentle, but never was it crowned for its gentleness: since its gentle quality belongs to it not from choice but by nature.
Of the Body.
Source: Catechetical Lectures (New Advent)