Barbara comes up with a sudden insight. Undershaft dismisses all the "morality mongering," and asks her to tell Cusins what power really means. Barbara confesses that up until yesterday she believed herself in the power of God, but her father revealed that she was in the power of Bodger and Undershaft.

Undershaft urges her to "scrap" a morality and religion that no longer as she would scrap old machinery. Barbara demands that he justify himself. "Cleanliness and respectability do not need justification," retorts her father. He has saved his people's—and family's—souls by providing for them and saved them from the worst of crimes, poverty. Work, food, shelter, and clothing save, he states, not the Bible.

Undershaft recalls his young as a poor east ender who one day pledged to a sacred commandment—"Thou shalt starve ere I starve"—and became "free and great." He chose to play the thief or murderer rather than the pauper or slave. Dangerous before he had his "will," he has become as a self-made millionaire a "useful, beneficent, kindly person." Rather than unscrupulously make virtues out of poverty and starvation, Undershaft understands them as the worst crimes. One should not preach at them but kill them.

Undershaft explains that killing is the "final test of conviction, the only lever strong enough to overturn a social system, the only way of saying Must." Democracy is but an illusion and only violence inaugurates the new. Undershaft has no patience with the moralist's "ought"—man must turn his "oughts" into "shalls." The history of the world is the history of those who courageously embrace this truth.

Britomart forbids Barbara to listen to her father and announces that the family is leaving. Barbara tells her it does no good to leave the wicked and Britomart counters that it shows one's "disapprobation." When Lomax moves to Undershaft's defense, Britomart pointedly notes that he only flatters him for his allowance. She condemns group, petulantly declaring her conscience is clear.

Undershaft demands that Cusins make his decision. He argues that Cusins overvalues Barbara, that his righteousness is patronizing, that his pity is the "scavenger of misery." As Plato says, society cannot be saved until the Professors of Greek make gunpowder or vice versa. Undershaft wants nothing of Cusins' impertinent gospel of love. Instead, he wants his obedience and respect on the pain of death. Undershaft loves only his best friend—that is, the "bravest enemy," or the man who keeps him "up to the mark." Cusins agrees.