Captain Ahab seeks revenge against the white whale that he perceives as a supernatural creature of malice.
At one point in his narrative, Ishmael describes Captain Ahab:
"While his live leg made lively echoes on the deck, every stroke of his dead limb sounded like a coffin-trap. On life and death this old man walked." (Ch. 51)
Ahab seeks the white whale with a monomaniacal desire for revenge. When the first mate, Starbuck, repudiates the idea of chasing Moby Dick, exclaiming,
"Vengeance on a dumb brute...that simply smote thee from blindest instinct! Madness! To be enraged with a dumb thing, Captain Ahab, seems blasphemous." (Ch. 36)
Ahab answers Starbuck's charges, saying that everything that is visible is but "as pasteboard masks." It is behind these masks that lies the unknown "but still reasoning thing[s]." Therefore, man must strike through these masks if he is to know what lies behind them.
"How can the prisoner reach outside except by thrusting through the wall? To me, the White Whale is that wall, shoved near to me. Sometimes I think there's naught beyond. But 'tis enough. He tasks me; he heaps me; I see in him outrageous strength, with an inscrutable malice sinewing it. That inscrutable thing is chiefly what I hate, and be the White Whale agent, or be the White Whale principal, I will wreak that hate upon him." (Ch.36)
Ahab perceives the white whale as a mysterious force of evil that he desires to conquer. He wants to "break through" to the supernatural force that lies behind the physical being of the whale and destroy this if he can.