Throughout his narrative in Joseph Conrad's Heart of Darkness, Charlie Marlow characterizes events, ideas, and locations that he encounters in terms of light or darkness. Embedded in Marlow's parlance is an ongoing metaphor equating light with knowledge and civility and darkness with mystery and savagery. When he begins his narrative, Marlow equates light and, therefore, civility, with reality, believing it to be a tangible expression of man's natural state. Similarly, Marlow uses darkness to depict savagery as a vice having absconded with nature. But as he proceeds deeper into the heart of the African jungle and begins to understand savagery as a primitive form of civilization and, therefore, a reflection on his own reality, the metaphor shifts, until the narrator raises his head at the end of the novel to discover that the Thames seemed to 'lead into the heart of an immense darkness.'' The alteration of the light-dark metaphor corresponds with Marlow's cognizance that the only 'reality', 'truth', or 'light' about civilization is that it is, regardless of appearances, unreal, absurd, and shrouded in 'darkness'.
Marlow uses the contrast between darkness and light to underscore the schism between the seemingly disparate realms of civility and savagery, repeatedly associating light with knowledge and truth; darkness with mystery and deceptive evil. When Marlow realizes that his aunt's acquaintances had misrepresented him to the Chief of the Inner Station, Marlow states, 'Light dawned upon me', as if to explicitly associate light with knowledge or cognizance. It is significant then, that Marlow later associates light with civilization. He describes the knights-errant who went out from the Thames to conquer the vast reaches of the world as having brought light into the darkness, flanked with figurative torches alongside their swords, 'bearers of a spark from the sacred fire." That Marlow directly correlates knowledge and light, and light and civilization, necessarily implies that Marlow seeks to correlate knowledge and civilization. In a word, Marlow's delineation of the British imperialists implies that he understands civilization to be logical and rational, while he understands primitive social organizations to be backward and crude.
As Marlow proceeds deeper into the heart of the African jungle and begins to understand savagery as a primitive form of civilization and, therefore, a reflection on his own reality, the light-dark metaphor shifts. For example, when Marlow goes wandering in the jungle, he has contrasting experiences in the sunshine and in the shade that are ironic in light of the established metaphor. Contemplating the colonialists in the jungle, he remarks:
'I've seen the devil of violence, and the devil of greed, and the devil of hot desire; but, by all the stars! These were strong, lusty, red-eyed devils, that swayed and drove men - men, I tell you. But as I stood on the hillside, I ...