These two figures exist in opposition to one another in this novel. Captain Ahab casts the white whale, Moby Dick, as his enemy and the whale seems to respond in kind. Beyond the formal representation of natural enemies, Ahab and the whale are laden with further symbolic meaning. This is especially true of the whale, whose meaning is discussed at length in the novel.
Moby Dick stands as a figure of Nature in the text, a creature of the unbounded sea.
In developing the theme of the individual (Ahab) versus Nature (symbolized by Moby-Dick), Melville explores the attributes of natural forces.
Moby Dick is therefore a representative of the chaos present in Nature. He is also emblematic of the philosophical "natural order" that Ahab resists and challenges, which places God and Nature above mankind, able to dictate man's fate.
Various anatomical descriptions are made of the whale, as the science of the day sought to understand the world of Nature and in that way gain some access to its inner-workings if not control of those mechanisms. These discussions are imbued with specific meanings that underscore the symbolic signficance of the whale.
The whale’s head thus symbolizes the unsympathetic and irresistible forces of nature.
If we reduce the pair in opposition in this novel to a simple idea, we can say that Moby Dick is a representative of Nature (or God) and Ahab stands for mankind.
The driving force behind Ahab's philosophical rebellion and his need for revenge is his insistence on free-will. He will not suffer the ignoble fate dictated to him by the whale; will not humbly accept the loss of his leg. Even if the whale kills him, Ahab will choose his own doom.
Thus, Ahab represents mankind in rebellion against Nature, fate, or God.
He will fight against fate, rather than resign himself to a divine providence.