The primary focus of the book is moral decay, with all other themes flowing toward it like tributaries into a river. Themes such as racism, madness, loneliness, deception, disorder, doubt, ambiguity, violence, and cruelty all lead to the moral deterioration represented by Kurtz's behavior in the Congo. Kurtz has forsaken reason, allowing his most primal and savage instincts to run rampant. He has permitted the evil within him to seize control. Kurtz's startling moral decline results not only from external influences, such as the isolation imposed by the jungle, but also, as Conrad implies, from internal forces within every individual, ready to emerge when given the chance. Kurtz may acknowledge the depth of his own moral decay when he cries out, "The horror! The horror!" on his deathbed. Marlow senses this realization passing to him and understands that he, too, could fall into moral corruption in a lawless setting. The journey uncovers the savage nature of humanity, not by traveling upriver, but by exploring one's own soul.