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[Marlow with the doctor]: "As we sat over our vermouths he glorified the Company’s business, and by-and-by I expressed casually my surprise at him not going out there. He became very cool and collected all at once. "I am not such a fool as I look, quoth Plato to his disciples," he said sententiously, emptied his glass with great resolution, and we rose." (1.25)
The doctor implies that going into the interior is something only a "fool" would do and suggests that the journey can only end badly. Anyone who starts it is only setting himself up for madness and defeat.
"The old doctor felt my pulse, evidently thinking of something else the while. "Good, good for there," he mumbled, and then with a certain eagerness asked me whether I would let him measure my head. Rather surprised, I said Yes, when he produced a thing like calipers and got the dimensions back and front and every way, talking notes carefully. He was an unshaven little man in a threadbare coat like a gaberdine, with his feet in slippers, and I thought him a harmless fool. "I always ask leave, in the interests of science, to measure the crania of those going out there," he said. "And when they come back too?" I asked. "Oh, I never see them, " he remarked; "and, moreover, the changes take place inside, you know." He smiled, as if at some quiet joke. "So you are going out there. Famous. Interesting too." He gave me a searching glance and made another note. "Ever any madness in your family?" he asked, in a matter-of-fact tone. I felt very annoyed. "Is that question in the interests of science too?" "It would be," he said, without taking notice of my irritation, "interesting for science to watch the mental changes of individuals, on the spot, but […]." (1.26)
The doctor implies that going into the interior changes men’s psyches and he tries, in vain, to measure their skulls before they leave. He eventually suggests that men have a tendency to go mad in the African interior. Marlow thinks this is all lunacy.
"The idleness of a passenger, my isolation amongst all these men with whom I had no point of contact, the oily and languid sea, the uniform sombreness of the coast, seemed to keep me away from the truth of things, within the toil of a mournful and senseless delusion. The voice of the surf heard now and then was a positive pleasure, like the speech of a brother. It was something natural, that had its reason, that had a meaning." (1.30)
Marlow’s isolation from other men and the unchanging scenery of the coast lulls him into a comforting and false sense of security. In retrospect, he knows that he was living a "senseless delusion" in which nature is "a positive pleasure" and even makes sense. The further he gets into the interior, the more he becomes disillusioned.