check out our:
"You can't understand. How could you? - with solid pavement under your feet, surrounded by kind neighbours ready to cheer you or to fall on you, stepping delicately between the butcher and the policeman, in the holy terror of scandal and gallows and lunatic asylums - how can you imagine what particular region of the first ages a man's untrammelled feet may take him into by the way of solitude - utter solitude without a policeman - by the way of silence - utter silence, where no warning voice of a kind neighbour can be heard whispering of public opinion? These little things make all the great difference. When they are gone you must fall back upon your own innate strength, upon your own capacity for faithfulness…The earth for us is a place to live in, where we must put up with sights, with sounds, with smells, too, by Jove! - breathe dead hippo, so to speak, and not be contaminated." (2.29)
Marlow claims that his audience cannot understand his feeling of utter loneliness and the ensuing madness without being there. He describes how isolation from one's fellow man can mess with one's sense of reality, that without public opinion, one cannot judge the morality of one's actions.
[The harlequin]: "'He [Kurtz] made me see things—things.'" (3.2)
We have to ask: did Kurtz happen to pass along any pharmaceuticals, too? Because if he's making the harlequin "see things" (unnamed things) just with words—that's pretty crazy. In all senses of the word.
"[…] as a rule Kurtz wandered alone, far in the depths of the forest." (3.4)
Kurtz willingly isolates himself from his friend, the harlequin—and by now, we know that isolation is a major warning sign for subsequent insanity. (We knew we needed to get out more.)