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"It was as unreal as everything else - as the philanthropic pretence of the whole concern, as their talk, as their government, as their show of work. The only real feeling was a desire to get appointed to a trading-post where ivory was to be had, so that they could earn percentages. They intrigued and slandered and hated each other only on that account—but as to effectually lifting a little finger - oh, no." (1.56)
Marlow is appalled by these pilgrims' depth of corruption. It seems utterly "unreal" to him that men could be so hypocritical. The unifying trait between them seems to be greed. (Nice words coming from someone who's heading to the Interior for profit himself.)
"No, I don't like work. I had rather laze about and think of all the fine things that can be done. I don't like work - no man does - but I like what is in the work - the chance to find yourself. Your own reality - for yourself, not for others - what no other man can ever know. They can only see the mere show, and never can tell what it really means." (1.68)
Marlow like to work because he can find his own version of reality in it. Nobody else, he claims, can see what a worker sees when he does his duty and claims the work as his own. Another can only see the external—the least true—account of reality. Um, if Marlow really is seeing different versions of reality, we're not sure that he's totally sane at this point.
"There were moments when one's past came back to one, as it will sometimes when you have not a moment to spare for yourself; but it came in the shape of an unrestful and noisy dream, remembered with wonder amongst the overwhelming realities of this strange world of plants, and water, and silence. And this stillness of life did not in the least resemble a peace. It was the stillness of an implacable force brooding over an inscrutable intention. It looked at you with a vengeful aspect." (2.5)
Marlow's very own comfy memories start to seem just as alien as the wilderness. This surrealism makes Marlow feel as though the jungle around them is alive and looking at him "with a vengeful aspect," and—as we all know—thinking that the world is out to get you is a pretty good sign of madness. (Usually. Unless there's a good reason they're after you.)