what is the dark or darkness as motif. in heart of darkness

Darkness is important enough conceptually to be part of the book’s title. However, it is difficult to discern exactly what it might mean, given that absolutely everything in the book is cloaked in darkness. Africa, England, and Brussels are all described as gloomy and somehow dark, even if the sun is shining brightly. Darkness thus seems to operate metaphorically and existentially rather than specifically. Darkness is the inability to see: this may sound simple, but as a description of the human condition it has profound implications. Failing to see another human being means failing to understand that individual and failing to establish any sort of sympathetic communion with him or her.

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The "darkness" is man's dark side, and what happens when someone allows darkness to take over them completely.