The main theme of Herman Melville's novel "Moby Dick" is the struggle between man and nature, particularly the obsession of Captain Ahab with the titular white whale. This theme explores the limits of human understanding and control in the face of the vast, indifferent forces of nature. Ahab's relentless pursuit of the whale serves as a metaphor for humanity's deeper existential battles, including revenge, fate, and obsession. Additionally, "Moby Dick" delves into complex themes of isolation, the duality of nature, and the ambiguity of good and evil, resonating with readers across generations.