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"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. They howled and leaped, and spun, and made horrid faces; but what thrilled you was just the thought of their humanity - like yours - the thought of your remote kinship with this wild and passionate uproar. Ugly. Yes, it was ugly enough; but if you were man enough you would admit to yourself that there was in you just the faintest trace of a response to the terrible frankness of that noise, a dim suspicion of there being a meaning in it which you - you so remote from the night of first ages - could comprehend." (2.8)
Marlow is so mixed up that he's even beginning to consider the wild screaming Africans to be human. (Crazy, right?) This means that he has to reformulate what falls in the boundaries of humanness. What he once thought of as savage is actually just part of being human.
"Let the fool gape and shudder - the man knows, and can look on without a wink. But he must at least be as much of a man as these on the shore. He must meet that truth with his own true stuff - with his own inborn strength. Principles won't do. Acquisitions, clothes, pretty rags - rags that would fly off at the first good shake. No; you want a deliberate belief." (2.8)
Weird. We usually think of principles as being pretty firm, but Marlow seems to think they're nothing more than "pretty rags." So, what's the difference between "principles" and "beliefs"?
"The dusk came gliding into it long before the sun had set." (2.13)
The sun may be up, but it's not light outside. In the interior, evil (or darkness, at least) seems to have a greater hold than on the outside.