As mentioned in the previously posted quotation, Ahab seeks "that intangible malignity" that he believes is embodied in the White Whale. In a separate chapter (42), in fact, Melville considers the whiteness of the whale as symbolic of evil. Even Ismael, the narrator, finds
It was the whiteness of the whale that above all things appalled me.....As for the white shark, the white gliding ghostliness of respose in that creature....This elusive quality....Now, in allusion to the white, silent stillness of deth in the shark, and the mild deadliness of his habits....Therefore, in his other moods, symbolise whatever grand or gracious thing he wll by whiteness, no man can deny that in its profundest idealised significance, it calls up a peculiar apparition to the soul.
It is this apparition of evil and mystery that Captain Ahab would not only avenge himself, but understand. Why is it white, Ishmael wonders, and why does it
appeal with such power to the soul, and more strange and far more portentous--why, as we have seen, it is at once the most meaning symbol of spiritual things, nay, the very veil of the Christians' Deity; and yet should be as it is, the intensify agent in things the most appalling to mankind.
It is the "invisible spheres" that Ahab seeks to comprehend in his revenge against the whale. When Starbuck tell him, "To be enraged with a dumb thing, Captain Ahab, seems blasphemous," Ahab replies,
Hark ye yet again--the little lower layer. All visible objects, man, are but as pasteboard masks. But in each event--in the living act, the doubted deed--there,, some unknown but still reasoning thing puts forth the moldings of its features from behind the unreasoning mask. If man will strike, strike through the mask! How can the prisoner reach outside except by thrusting through the wall? To me, the White Whale is that wall, shoved near to me....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...I will wreak that hate upon him....Truth hath no confines.
The preternatural White Whale wears the "pasteboard mask" that Captain Ahab is obsessed with striking through. He would know what metaphysical meaning lies behind this creatures eye that cannot see before him, but only sideways from his head. Revenge against him for his lost leg is a small part of what Ahab seeks; the Pequod's voyage is a metaphor for life and Ahab is man searching for meaning.
On a previous voyage, Ahab lost his leg to Moby Dick, and his life was changed forever. Little else is known of the captain before he took charge of the Pequod, except that he is a Quaker. But contrary to the peace loving precepts of the religion, Ahab is driven to near madness by his obsession with the beast that maimed him.
In the larger context, perhaps Ahab represents the rage in all men against the injustices of life and of the bitter imbalance between good and evil, in reality or in the mind:
The White Whale swam before him as the monomaniac incarnation of all those malicious agencies which some deep men feel eating in them, till they are left living on with half a heart and half a lung. That intangible malignity which has been from the beginning; to whose dominion even the modern Christians ascribe one-half of the worlds; which the ancient Ophites of the east reverenced in their statue devil; -- Ahab did not fall down and worship it like them; but deliriously transferring its idea to the abhorred white whale, he pitted himself, all mutilated, against it. All that most maddens and torments; all that stirs up the lees of things; all truth with malice in it; all that cracks the sinews and cakes the brain; all the subtle demonisms of life and thought; all evil, to crazy Ahab, were visibly personified, and made practically assailable in Moby-Dick. He piled upon the whale's white hump the sum of all the general rage and hate felt by his whole race from Adam down; and then, as if his chest had been a mortar, he burst his hot heart's shell upon it.
And here, in Ahab's own words, we see the extent of his anger, rage and vehemence towards the creature he battled with, inside himself and out:
... to the last I grapple with thee; from hell's heart I stab at thee; for hate's sake I spit my last breath at thee.
Ahab seeks revenge for having lost his leg; he also has a long scar running down the length of his face. He offers a sixteen-dollar gold piece (quite a lot of money at the time) to the first man aboard who spots the whale.
Ahab makes the mistake of interpreting his injuries in a whaling accident as a personal conflict between the whale and himself. Not only does he give the whale a name, but he attributes it an anthropomorphic dimension. Moby Dick becomes his personal enemy against "whom" he develops a obsessional grudge. Paying back Moby Dick by finishing "him" off in an ultimate confrontation becomes Ahab's unique goal in life, as his whole identity and reason for being revolves around this "quest."
Herman Melville, a whaler himself, was a simple man but he had certain insight into human psychology. Through the raging and ranting of his protagonist Ahab, Melville demonstrates that life is what you make of it and that often "problems" are not really circumstantial but rather fixations within the mind. Today Ahab would be a prime candidate for treatment for obsessive compulsive disorder (OCD syndrome). See the following reference for further information concerning this very particular personality disorder.