How do the characters in The Sun Also Rises show that modernists do not subscribe to absolute truth and that for them all things are relative?

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.