Poe’s Perversity

In the “Imp of the Perverse” Poe divides his story into two parts: an essay and short story.  In the short story the narrator confesses to his crime without coercion.  In the essay, the narrator argues that even the intellectual or logical man has missed one’s propensity to inflict harm on one’s self, “We have suffered its existence to escape our senses, solely through want of belief–of faith;…This idea of it has never occurred to us, simply because of its supererogation.  We saw no need of the impulse–for the propensity”. 

What is the narrator grappling with in both the essay and short story?