How do you understand the White Whale or Moby Dick in Herman Melville's Moby-Dick?

How do you understand the White Whale or Moby Dick in Herman Melville's Moby-Dick?

The white whale. Moby Dick, symbolizes nature for Melville, for it is complex, unfathomable, malignant, and beautiful as well. For the character Ahab, however, the whale represents only evil. Moby Dick is like a wall, hiding some unknown, mysterious things behind. For the author, as well as for the reader and Ishmael,Moby Dick is still a mystery, an ultimate mystery of the universe, inscrutable and ambivalent,and the voyage of the mind will forever remain a search, not a discovery, of the truth. (P461)