In analysis of Heart of Darkness, much is made of Conrad's intentions in telling his tale. People search for a moral lesson, a strict social commentary, an absolution for the evil of the dark jungle. It isn't there, and that's not the point.
In works of philosophy (like The Republic), or works of political theory (like Socialism: Utopian and Scientific), or works of natural science (like The Origin of Species), this sifting of important and clear ideas from the mess and confusion of experience is what writers like Plato, Darwin, or Engels are doing. They experience the world in all its messy confusion, and then they attempt to abstract from the mess, by careful selection, a system of ordering principles which other people can comprehend and make use of. In more figurative words, they are trying to shed the light of intelligence upon the darkness of experience.
As, primarily, students and teachers, we naturally look for the conveyance of such ideas in any material we encounter. We miss that books like Heart of Darkness are fundamentally different in intent and we continue searching for that lesson from which to make a rational response to the story.
Even literary professionals seem often to fall into the error of neglecting or misunderstanding the novelist's purpose. Consider, for example, the criticism leveled against Heart of Darkness by Paul O'Prey in his introduction to the Penguin edition. He writes:
"It is an irony that the failures' of Marlow and Kurtz are paralleled by a corresponding failure of Conrad's technique--brilliant though it is--as the vast abstract darkness he imagines exceeds his capacity to analyze and dramatize it, and the very inability to portray the story's central subject, the unimaginable', the impenetrable' (evil, emptiness, mystery or whatever) becomes a central theme."
Mr. O'Prey's sentence is somewhat impenetrable itself, but his complaint is that Conrad wants to evoke an abstract notion...