“With regard to evil thoughts, there may be a twofold delusion. God-fearing souls who have little or no gift of discernment, and are inclined to scruples, think that every wicked thought that enters their mind is a sin. This is a mistake, for it is not the wicked thoughts in themselves that are sins, but the yielding or consenting to them. The wickedness of mortal sin consists in the perverse will that deliberately yields to sin with a complete knowledge of its wickedness with full consent. And therefore St. Augustine teaches that when the consent of the will is absent, there is no sin. However much we may be tormented by temptations, the rebellion of the senses, or the inordinate motions of the inferior part of the soul, as long as there is no consent, there is no sin. For the comfort of such anxious souls, let me suggest a good rule of conduct that is taught by all masters in the spiritual life. If a person who fears God and hates sin doubts whether or not he has consented to an evil thought or not, he is not bound to confess it, because it is morally certain that he has not given consent. For had he actually committed a mortal sin, he would have no doubt about it, as mortal sin is such a monster in the eyes of one who fears God that its entrance into the heart could not take place without its being known. Others, on the contrary, whose conscience is lax and not well-informed, think that evil thoughts and desires, though consented to, are not sins provided they are not followed by sinful actions. This error is worse than the one mentioned above. What we may not do, we may not desire. Therefore an evil thought or desire to which we consent comprises in itself all the wickedness of an evil deed.”
—St. Alphonsus Liguori,