This was written for my 3rd year college film noir course. It presents an interesting new lens through which to view the plots of noir films.
Film Noir is presented to us as a series of decisions. We are not however clearly presented with the corresponding motivations. In this way Noir imitates life, in that the knowledge of the present does not imply a knowledge of the past.
But without justifications, an observer's sense of moral responsibility is impossible. Fortunately, the characters in Film Noir are painted with bold (if monochrome) strokes, so we may attempt to extrapolate a moral system to provide reasons for what we see.
Unfortunately, Noir is also a world of pragmatism and ubiquitous malice. So when we see Marlowe tear into the desert after a loose end of a case that he's completed and been paid for, we get the sense of an obligation beyond that of mere survival. Every once in a while, a character believes that certain aspects of the noiriverse must be “corrected” in accordance with some perfect and objective ideal.
Thus Noir in fact presents a dialectic of sorts. At one pole is the practicality of the streets and the cruelty it engenders. Opposing this is a human sense of right and wrong. The tension between these foci generates Noir's primary conflict, that of responsibility. This tension is felt even within the overt character conflicts in the films. The best example of this is Bogart's final scene with Brigid, where Bogart's decision to have her arrested orbits and orbits these poles.