As Marlow describes his travels up the Congo River toward the Inner Station, he uses a pair of oxymorons to capture the mix of confusion and horror that he feels as he watches native people on the riverbanks and reflects on his participation in colonial exploitation:
The earth seemed unearthly. We are accustomed to look upon the shackled form of a conquered monster, but there—there you could look at a thing monstrous and free. It was unearthly, and the men were—No, they were not inhuman. Well, you know, that was the worst of it—this suspicion of their not being inhuman. It would come slowly to one.
By “the shackled form of a conquered monster,” Marlow is referring to Congolese people whom Europeans have enslaved. And since these are the only Congolese people he has seen until now, he is surprised to see the “monstrous and free” communities living on the riverbanks. Marlow uses his first oxymoron, “the earth seemed unearthly,” to capture how seeing these native communities challenges his entire sense of reality. It turns his perceptions about the Congo and his feelings about European colonialism on his head because it shows him that he is a foreign invader, entering other people’s territory to seize their resources and enslave them. Marlow’s second oxymoron is implicit: he nearly says that the native people are “inhuman,” but he cuts himself off before doing so, because he knows that no human being can ever truly be “inhuman.” Instead, he euphemistically acknowledges that Congolese people are human beings by calling them “not inhuman.”
Of course, on some level, Marlow has always known that all Congolese people are fully human. But he has tried to block this truth from his mind because it is too frightening. It suggests that Africans are just as human as Europeans and are therefore capable of everything that Europeans can do. In turn, this sneaking “suspicion of their not being inhuman” reminds Marlow that he is participating in a profoundly evil system that strips people of their lives, dignity, and humanity—and he must accept his choice’s the moral consequences.