Big Sur makes rather explicit comparisons between the city (San Francisco, specifically) and the natural world (in this case, a cabin in the woods of Big Sur, California). Kerouac describes the city as "gooky," confining, a place in which he feels "trapped," a place he needs to "escape." Big Sur, while freeing, is also frightening. The natural world is incredibly large, and serves as a reminder of the insignificance of man and the futility of one's life. Narrator Jack Duluoz finds both environments problematic for these very different reasons.
Jack's breakdown at the end of the novel is the result of worlds colliding; he tries to mix his social and sexual life with his solitary and spiritual ones.