As a creature who is part of the vast universe that has many meanings, so too does Moby Dick have different meanings for different characters in Melville's novel. These different meanings serve to develop the motif that the singleminded pursuit of an ideal by man is both vain and destructive. Thus, the pursuit of Moby Dick, the great white whale, is a metaphor for the conflict between humanity and the power of nature, between man and his own fate.
In Chapter 41 as the whalers pursuit Moby Dick and encounters become more frequent, rumors abound. The whale is purportedly ubiquitous. Others say he is immortal; Ahab contends that he has "an inscrutable malice." Likewise, the interpretation of the meaning of his whiteness varies. Some perceive it as symbolic of death, for Death rides a white horse, but others perceive white as symbolic of innocence. Ishmael finds white most disturbing as the absence of color: blankness.
At any rate, the great whale is a metaphysical force of nature that knows the depths of the sea, a knowledge man can never attain. The whiteness of the whale can signify this absence of meaning in the universe amid the heaving of the black sea, symbolic of man's conscience. Of course, for Captain Ahab, reality merges with his conception of it. Therefore, it does not matter what the real physical status of the whale is; for him the symbolism he has assigned it becomes all important.