The saying in Matthew 5:27-28 (“You have heard that it was said, ‘You shall not commit adultery.’ But I say to you that everyone who looks at a woman with lust has already committed adultery with her in his heart.” NRSVCE ) addresses both the objective and subjective aspects of adultery. As part of the Torah, the Ten Commandments are the foundation of the Mosaic law. In the “Sermon on the Mount”, Jesus refers to this objective standard from Revelation, as quoted in Matthew 5:27. Pope John Paul II then emphasizes Jesus’ focus on the subjective, interior part of the commandment, rather than on the violation of “property rights” of a woman’s husband: "This statement is one of the passages of the Sermon on the Mount in which Jesus brings about a fundamental revision of the way of understanding and carrying out the moral law of the Old Covenant” (John Paul II, “Man and Woman He Created Them”, Section 24:1) .
This is significant because the Sermon on the Mount emphasizes the personal rather than simply the objective. As we have seen emphasized in “Love and Responsibility”, man’s interiority is what makes him a person: “It is remarkable that precisely through his interiority and interior life man not only is a person, but at the same time most inheres in the objective world” (Karol Wojtyla, “Love and Responsibility” 2nd edition, p. 5). In this saying, Jesus requires his disciples to imitate him in their sexual ethics in the transcendence that is only possible for persons: “The morality in which the very meaning of being human is realized—which is, at the same time, the fulfillment of the law by the ‘super-abounding’ of justice through subjective vitality—is formed in the interior perception of values, from which duty is born as an expression of conscience, as an answer of one’s own personal ‘I.’ Ethos makes us, at one and the same time, enter into the depth of the norm itself and descend into the interior of man, the subject of morality. Moral value is connected with the dynamic process of man’s innermost [being]. To reach it, it is not enough to stop “on the surface” of human actions, but one must penetrate precisely the interior.” (Ibid., Section 24:3).
Thus, Jesus is calling his disciples into the “depth of the norm” through descending into the “interior of man”. How does this apply to sexual ethics? Certainly, this requires attention to one’s “heart”: “The ‘heart” has become a battlefield between love and concupiscence. The more concupiscence dominates the heart, the less the heart experiences the spousal meaning of the body, and the less sensitive it becomes to the gift of the person that expresses precisely this meaning in the reciprocal relations of man and woman.” (Ibid., Section 32.3). Through the “concupiscence of the eyes”, a man enters into a non-spousal relationship with a woman as he has “reduced” her to an object, an object of his lust. This denial of the woman’s interiority leads the man and woman into a sexual relationship that is not marital, thus instantiating a relationship that is “adulterous” compared to the marital relationship that is or could be: “The relationship of the gift changes into a relationship of appropriation” (Ibid., Section 32.6).
Entering into the “new ethos” of the Matthew 5:28 requires that the man take responsibility for his relationship with the woman, to protect her from his “reductive” desires that denies a spousal relationship, and vice versa. He must guard his heart against the “lust of the eyes”: “Christ makes the moral evaluation of ‘desire’ depend above all on the personal dignity of the man and the woman; and this is important in the case of unmarried persons and—perhaps even more so—in the case of spouse, husband and wife” (Ibid., Section 42:7). John Paul II speaks of “purity of heart” as the goal and foundation for relationships, a position not of “using” the other but where “human beings cannot share without firmness in facing everything in its origin in concupiscence of the flesh ‘Purity of heart’ is gained by the one who knows how to be consistently demanding from his heart’ and from his ‘body” (Ibid., Section 43:5). This then interconnects his actions and his subjectivity in a way that focuses on the spousal relationship and the dignity of woman.