Because most of the action of the novel takes place aboard ship, it is not surprising that duty is a major theme in Moby-Dick. The problem is how it is to be interpreted. For Father Mapple, the first duty of any shipmate is to God. We can serve our professional obligations only within that larger value system. This is not the case with Ahab. After Ahab's initial disagreement with Starbuck on the quarter-deck (Chapter 36) regarding the ship's mission, the crew sees Ahab as its highest authority. Later in the voyage, Ahab and Starbuck have another confrontation, again concerning duty, in the captain's cabin (Chapter 109).
Starbuck is a sincere Quaker with a hierarchy of loyalties: He feels a duty first to God, then to his employer (who supports Starbuck's family), then to his captain. When Starbuck discovers that some of the barrels in the hold of the ship must be leaking oil, he reports the situation to Ahab. The first mate expects the captain to stop the ship and turn all hands to a check of the casks because the ship's official mission is to capture whale oil and bring it home safely. As he says, "What we come twenty thousand miles to get is worth saving, sir." Ahab sardonically responds, "So it is, so it is; if we get it." Starbuck means the oil; Ahab means the White Whale. Starbuck reminds Ahab of the owners' interests, but the captain could not care less about the owners. He points a loaded musket toward the first mate and declares that there is "one Captain that is lord over the Pequod." Starbuck returns to the deck, and Ahab soon decides it is more prudent to stop the ship and make repairs.
It is clear, however, that the captain feels only one duty on this mission, and that is not to the owners or even to God but to Ahab. He will pursue his own monomaniacal goal in defiance of whatever gets in his path. The only way to stop Ahab is to kill him. When Starbuck has an opportunity to shoot the old man, with the same musket that Ahab pointed at him, the duties become confused in the first mate's mind. He has a duty to his family. How is that duty best served? He has a duty to the men who may well die with Ahab. But Starbuck feels a higher duty — to himself, to God, perhaps simply to decency. He is unable to pull the trigger, not through weakness but due to his own system of values. Because Starbuck cannot kill his captain, he must serve him.
