1. The Ten Commandments forbid not only evil deeds but evil desires (see Ex 20.17; Dt 5.21). In stating that love fulfills the whole law, St. Paul includes the sin of coveting as part of the law (see Rom 13.9). Thus evil desire violates the law of love, even though it does no outward damage. Jesus, deepening the commandments in the Sermon on the Mount, emphasizes that sins of murder and adultery are already committed when one nurses anger and lustful thoughts (see Mt 5.22, 28).7
In commenting upon the Sermon on the Mount, St. Augustine describes the psychology of sin which begins in the heart: “For there are three steps in the commission of sin: suggestion, pleasure, consent. Suggestion comes about either through memory or a sense perception as when we see, hear, smell, taste or touch anything. If to enjoy any of these sensations brings pleasure, the pleasure, if forbidden, must be checked. . . . Were we to yield consent to it, we would commit sin surely, a sin in the heart known to God, though actually it may remain unknown to man.”8 Augustine goes on to explain that if a habit has not been formed, the pleasure is less intense and more easily resisted. If one carries through and puts the consent in the heart into action, desire at first seems satisfied, but a habit is formed, and pleasure becomes more intense and harder to resist.9 Thus, on Augustine’s account, the sinful deed must be avoided more for the sake of preventing sins of thought than sins of thought must be avoided for the sake of preventing sinful deeds.
2. 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 (see Mt 23.25–28; see S.t., 1–2, q. 74, a. 1). 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).
3. In its definitive teaching on the sacrament of penance, the Council of Trent explicitly teaches that even completely interior sins, which violate only the last two of the Ten Commandments, can be mortal and must be confessed (see DS 1707/917). Indeed, the Council teaches that these sins “sometimes wound the soul more grievously and are more dangerous than those sins which are committed openly” (DS 1680/899).
4. Evil’s moral significance lies not so much in the harm done in outward fact as in the privation introduced in the existential domain (see S.t., 1–2, q. 73, a. 8, ad 2).10 This privation is less obvious but just as real in sins of thought as in gross, outward immorality. Morally evil choices mutilate sinners, and this mutilation at once and of itself brings disharmony into their relationships with other people and God. As soon as a man commits adultery in his heart, for instance, his relationship with his wife is damaged and so is his relationship with Jesus, in which the sacramental marital relationship participates.
5. One who tries to avoid sinful outward behavior while freely indulging in grave sins of thought inevitably takes a false, legalistic attitude toward morality. If one’s heart is not pure, the attempt to avoid impure behavior becomes a pharisaic pretense. In such a case outward conformity to moral standards can only be the result of a nonmoral motive, such as shame or fear of punishment. Moral standards will seem arbitrary, irrational impositions, while inward love of goods and the attitude of openness toward integral human fulfillment will be lacking.
Inaccurate teaching concerning sins of thought can lead to morbid self-consciousness, inappropriate anxiety and feelings of guilt, and an inversion of the priority from doing good to avoiding evil. But to ignore or condone sins of thought is to undermine the inwardness of Christian morality, to encourage pharisaism, and ultimately to pave the way for a total abandonment of Christian moral standards in the interest of “honesty”—that is, the reintegration of people’s covetous hearts and their outward behavior. The only remedy is timely, careful, and accurate teaching about sins of thought. In this area there is much work to be done, because in times past instruction about sins of thought often was vague and confused, and in recent years it often has been omitted or lax.
7. For a good summary of scriptural teaching relevant to sin of thought, see H. Schönweiss, “epithymia,” New International Dictionary of New Testament Theology, 1:456–58.
8. St. Augustine, The Lord’s Sermon on the Mount, trans. John J. Jepson, S.S. (New York: Newman Press, 1948), 43.
9. Ibid., 44.
10. With the acceptance of proportionalism and its emphasis on what is brought about in making and executing choices, many Catholic moralists have tended to ignore or condone sins of thought. See, for example, Philip S. Keane, S.S., Sexual Morality: A Catholic Perspective (New York: Paulist Press, 1977), 46–51 and 57–59.