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"He [Kurtz] hated all this, and somehow he couldn't get away. When I had a chance I begged him to try and leave while there was time; I offered to go back with him. And he would say yes, and then he would remain; go off on another ivory hunt; disappear for weeks; forget himself amongst these people - forget himself - you know. 'Why! he's mad,' I said." (3.4)
Even though Kurtz "hates all this," he won't leave it willingly. And that, somehow, is the final clue that Marlow needs to decide that Kurtz is totally crazy. (Really, Marlow? We got there a lot faster than you.)
"Kurtz—Kurtz—that means short in German—don't it? Well, the name was as true as everything else in his life - and death. He looked at least seven feet long." (3.9)
As true as everything else, which means … complete false. One of the biggest signs of madness in Heart of Darkness is the way that language and meaning don't match up at all—in fact, they're so at odds that we end up with no meaning at all.
"Sometimes I ask myself whether I had ever really seen him—whether it was possible to meet such a phenomenon! […]." (3.22)
The harlequin is so hilariously weird that Marlow can't help wondering if he was really real, or if he was just some deranged hallucination of the interior. We have to say, it doesn't seem impossible that Marlow just imagined him.