Heart of Darkness develops a journey motif, and one of the journeys Marlow undertakes is a "night journey" into what has been called the primal self. He leaves London seeking knowledge of himself, and discovers more than he ever imagined as he is drawn into Kurtz' being. As he travels deeper and deeper into the Congo, Marlow leaves civilization behind. He enters what could be described as a dream state, which is reinforced through the novel's imagery of wilderness and fog. He is moving through a primal forest as he grows closer to his primal self. Through his developing relationship with Kurtz, Marlow descends psychologically into his deepest self where he finds that he, too, has a capacity for evil. Unlike Kurtz, however, Marlow's psychological self is developed and integrated; he makes a conscious choice to reject what he judges to be human degradation. Marlow's conscience, his super ego, recognizes evil and will not embrace it.
Kurtz is often interpreted as representing the bottom of the human psyche, the Freudian id, where instinct, impulse, and the most primitive of human needs exist. Kurtz is brutal, self-indulgent, and self-obsessed; his behavior is unchecked by conscience or society. Marlow notes that Kurtz "lacks restraint" and that there is "something wanting in him." As representative of the id as Freud defined it, what Kurtz lacks is judgment and a conscience. He does not operate from the ego or the super ego, as defined by Freud.