Q. How does one know when one has consented to an impure thought? What is the best way to keep impure thoughts from coming into your mind? Is one guilty of impure thoughts by overhearing other people’s crude or explicit conversations? – L.C., State Unknown

A. There is a story about a man who went to Confession and said, “Fr., I had impure thoughts.” “Did you entertain them?” the priest asked. “No, Father, but they sure entertained me.” The point of this exchange is that impure thoughts can enter anyone’s mind, especially in a society that exploits sex to sell everything from apples to zucchini. But we violate the Ninth Commandment only if we decide to keep those thoughts in our minds and take pleasure from them. No matter how much we are troubled by sexual images and ideas, even those overheard from other people’s conversations, they are not sinful unless we choose to sit back and enjoy them. “If one deliberately chooses to fix his desire on something that is gravely sinful,” said St. Thomas Aquinas, “it is a mortal sin.”

In his book Christian Moral Principles (Franciscan Herald Press), Professor of Christian Ethics Germain Grisez offers some good advice on the dangers of entertaining sinful thoughts (pp. 369-370):
“One must avoid evil thoughts because they are the beginning of evil deeds; to refrain from the deeds, one must nip the thoughts in the bud. There is, however, a more profound reason for emphasizing the morality of thoughts. Morality essentially pertains to thought; evil is much more in the heart than in outward behavior. Jesus emphasizes that the moral distinction between clean and unclean cannot be drawn by legalistic standards for outward behavior; rather, impurity emerges from the heart (see Mt 15:17-20; Mk 7:18-23).”

Grisez is echoing the warning given by Jesus in, for example, Mark 7:18-23:

“What emerges from within a man, that and nothing else is what makes him impure. Wicked designs come from the deep recesses of the heart: acts of fornication, theft, murder, adulterous conduct, greed, maliciousness, deceit, sensuality, envy, blasphemy, arrogance, an obtuse spirit. All those evils comes from within and render a man impure.”

What do we do to avoid impure thoughts? First, we must avoid persons, places, reading material, films, and TV shows that provoke such thoughts. If they still assail us, we must resist them firmly and fight them vigorously; we must think about something else, do something different to clear our minds. We must pray fervently to God and to the Blessed Virgin Mary to help us get rid of these thoughts. We must also take advantage of the Sacraments of Penance and the Holy Eucharist to give us the strength to say no to evil images and ideas.