Conrad wrote the Heart of Darkness when Europeans were engrossed with Imperialism. Interestingly, the character of Kurtz goes mad when his light goes out. The light is symbolic of the civilized side of human nature in society. In contrast, the dark represents the savage or uncivilized side of our society and individual natures. In the dark, Kurtz begins to question the value of European society, and civilization. This is a direct challenge to the principles of the Enlightenment.

Conrad states, "The brown current ran swiftly out of the heart of darkness..." This describes the savage and uncivilized nature of the Congo that Marlow and Kurtz are attempting to escape. Interestingly, Kurtz is broken...

In contrast, staring into the heart of darkness enlightens Marlow.
In general, in the Communist Manifesto, Karl Marx rejected the philosophy of the Enlightenment (and indeed, philosophy in general). Given Marx's ideological leanings, it is reasonable to assert that he likely viewed the Enlightenment writers who influenced him in his youth as members of the bourgeoisie. Interestingly, the reasoning that characterized Enlightenment texts can also be seen in Marx's the Communist Manifesto. Both the Communist Manifesto and writings of the Enlightenment are generally all systematically thought out and meticulously written.