The major theme of the novel that the symbol of the whale itself supports is the limits of human understanding. At the beginning of the novel the narrator tries to discuss the topic of whales and what they represent, but his task is made impossible by the multiplicity of meanings that whales have had through the centuries and also the way that man's knowledge of them is so limited, because they cannot see beneath the sea. In the same way, the novel as a whole points towards the vast realms of understanding and knowledge that humans are not aware of.
This is highlighted through the whale in the way that so many meanings are attached to it. For the sailors, the whale is a symbol of their fears and the danger they face as sailors. For Ahab, of course, the whale represents something completely different:
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.
The fact that the whale becomes a kind of repository for the feelings, fears and neuroses of other characters suggests again the theme of the limits of human knowledge, as characters seem to push on to the whale their fears of the unknown.