Melville's novel addresses several esoteric and intellectual ideas, beginning at the sermon early in the novel and continuing to the novel's conclusion. These ideas relate mainly to (1) an identification made between the sea and Nature (or God) and (2) the idea of free-will versus determinism.
Examples abound in the novel drawing the comparison between the sea and Nature. The sea represents absolute nature. It possesses the traits of both order and chaos, as do its creatures.
In developing the theme of the individual (Ahab) versus Nature (symbolized by Moby-Dick), Melville explores the attributes of natural forces.
As Ahab challenges the power of Nature (or God), he is also challenging the notion that god can determine his fate. Starbuck attempts to persuade Ahab to give up his quest. He is attempting to confront the divine, which is a kind of blasphemy to Starbuck. The ship will be doomed along with Ahab.
The debate as to the possibility of this confrontation underscores the practical question of the quest as the Pequod has difficulty finding a single whale in the expanse of the sea. Yet Ahab's will is bent on his aim. He pits himself against the whale and, importantly, against the intellectual idea that the whale represents.
He will fight against fate, rather than resign himself to a divine providence.