Brett and Jake from Hemingway's The Sun Also Rises not only demonstrate Modern American writers' disillusionment with life but also illustrate their subscription to relative truth and morality. The novel follows the main characters as they wander aimlessly around Europe, hurting one another, feeling some remorse, and then repeating their actions. Because Brett and Jake and others possess their own versions of reality (truth), they find themselves embroiled in internal conflict, not knowing if what they perceive is real or simply their perception of a situation--if it is only their perception, then it does not really matter how they act or what they think; at least, that is their only source of comfort when regret starts to creep in.
Hemingway clearly illustrates relative morality through Jake's character. His idea of right and wrong is based on what helps or pleases himself. Similarly, Brett's choices and skewed version of what she should and should not do stem from what will advance her own agenda. Because Cohn cannot figure out why something is right in one instance and wrong in another (according to Jake), he is an outsider and cannot truly understand the main characters.
In the end, the characters' dependence on relative truth and morality serves to highlight Hemingway's and other modern writers' disgruntled feelings toward traditional values and morals.
Excellent reply! Thank-you so much.