When the Doctor tells Marlow, "The changes take place inside, you know," he refers to the psychological transformations Marlow will experience, which cannot be measured scientifically. This statement critiques Enlightenment thinking and the now-discredited practice of phrenology, which attempted to assess character traits through skull measurements. Conrad uses this exchange to highlight the limitations of scientific inquiry in understanding the profound internal changes individuals undergo.

Marlow refers to England as "one of the dark places of earth" to highlight its transformation from a mysterious and uncivilized land to a dominant empire. He draws a parallel between ancient England, as seen by the Romans, and the Congo, suggesting that civilization and darkness are relative concepts. This reflects Conrad's broader theme of imperialism and the moral ambiguity of "civilizing" missions, questioning the true nature of civilization and savagery.

The phrase "the heart of darkness" refers to the inherently corrupting power that a person's environment might have on him or her. In other words, Conrad suggests that the corruption of society can be blamed on the society itself, and not necessarily the individual actors who inhabit it.

Heart of Darkness is about the abuse of power and how it affects the physical and mental states of both the oppressors and the victims. The environment symbolizes the moral degradation of both parties, but it also provides a setting to express these ideas. The theme: Joseph Conrad’s novel Heart of Darkness is an exploration into one man’s journey up a river in Africa. He takes us through a narrow channel where the sun seems to lose its light and joy, where even nature itself becomes “bewitched” and “cut off from everything you have known once—somewhere—far away—in another existence perhaps.

Marlow's amazement at the natives' restraint from attacking and eating the outnumbered white men highlights the unexpected civility of the "savages" compared to the reckless behavior of the "civilized" whites. This irony underscores Marlow's evolving perception of African natives as possessing admirable qualities, contrary to his initial prejudices. Conrad uses this reversal to critique European imperialism and challenge conventional racial stereotypes, illustrating a profound shift in Marlow's worldview.

In "Heart of Darkness," light and dark imagery is used to subvert traditional associations, where typically light represents goodness and dark represents evil. Here, the "light" English colonizers are portrayed negatively, while the "dark" African natives are victims. The terms "dark" and "darkness" differ in that "darkness" symbolizes the moral corruption of the colonizers, whereas "dark" is a descriptor of the African people's skin color, highlighting the racial and moral complexities.

Joseph Conrad explores the corruption of the human soul through Kurtz's transformation in the Congolese jungle. Initially depicted with noble intentions, Kurtz becomes a tyrant, idolized as a deity by the natives while committing brutal acts for ivory. His infamous phrase "Exterminate all the brutes!" highlights his moral decay. Marlow's journey parallels this, revealing mankind's inherent darkness. Kurtz's final words acknowledge his corrupted soul and the horrors he perpetrated.

The quote highlights the symbolism of light and darkness in the novel's final section, where Marlow visits Kurtz's Intended. The light on her forehead symbolizes her unyielding belief in Kurtz and colonial ideals, contrasting with the surrounding darkness that represents ignorance. Marlow's silence and the growing darkness emphasize her lack of awareness about Kurtz and colonialism's true nature. This imagery underscores the seductive power of grand narratives and the blindness they can foster.

Heart of Darkness critiques the ethical certainties and dark aspects of European civilization, particularly in the context of colonialism, by exposing the brutality and moral ambiguity of European imperialism in Africa. Through Marlow's journey, Conrad reveals the dehumanizing effects of colonization on both the colonizers and the colonized, challenging the notion of European superiority and civilization.

In Heart of Darkness, light symbolizes civilization, knowledge, and enlightenment, while darkness represents savagery, ignorance, and evil. Conrad uses these contrasts to explore the complexities of human nature and colonialism. The journey into the African Congo symbolizes a descent into the darkest parts of the human psyche, revealing the thin veneer of civilization and the inherent darkness within humanity.