In what dual sense is Marlow "penetrating deeper and deeper into the heart of darkness"? What do the circumstances of his journey contribute to his assertion that human beings need "a deliberate belief"?

O n this river Marlow starts his personal story which reminds him of his journey through the Congo, and of all the terrible things he has seen. Marlow associates light with knowledge and darkness with mystery and savagery. When he starts narrating, he equates light with civility, believing it to be natural, but when he gets deeper into "the heart of darkness" which is one of the most important metaphors in the book for the African jungle, he begins to understand, that savagery is a primitive form of civilisation.