Ought! Ought! Ought! Ought! Ought! Are you going to spend your life saying ought, like the rest of our moralists? Turn your oughts into shalls, man. Come and make explosives with me. Whatever can blow men up can blow society up. The history of the world is the history of those who had the courage to embrace this truth.
Undershaft exhorts Cusins to turn his "oughts" into "shalls" toward the end of Act III in urging him to surrender himself to the Undershaft tradition. The passage is also important in understanding Undershaft's gospel.
Here Undershaft explosively poses himself, the military industrialist, against common moralist as a man of action. What marks the transformation of the conditional "ought" into the imperative "shall" is the murderous power of gunpowder. As Undershaft declares earlier, the power of "killing" serves as his "final test of conviction," the "only way of saying Must" and imposing one's will. The man who can blow up armies has the power to blow up society. Only he can make history.