What is the theme of The Sun Also Rises?

Clearly, any work of literature has a number of themes,
and this novel is no exception. However, one clear theme to me is the way the novel
explores morality and the various moral codes that the characters in this novel ascribe
to. Let us consider Jake for a moment, who describes "morality" as "things that made you
disgusted afterward." He quickly corrects himself, saying that this must be the
definition of "immorality." However, throughout the novel it is clear that Jake is more
interested in his own problems and issues, and then, only secondly, those of
Brett.

Now, if we compare Jake's moral code to that of
Cohn, there is a stark difference, as Cohn shows himself unable to understand the
morality of Brett and also unable to follow it. Note how Brett and Romero are got
together, deliberately going against the morality of the group. This of course leads to
a betrayal and the loss of respect between Jake and Montoya and Cohn. Jake shows how
throughout the novel he continually becomes more and more self-centred and
selfish.

If we consider Brett for a moment, we see that she
is presented as wandering in a kind of moral vacuum. She shows self-disgust and also
disgust with Jake, even though this is shown to be unfair. She stands against
conventional morality by engaging in numerous brief affairs which shows her to be a
self-destructive personality that is often presented as being passive in the novel.
These string of affairs are of course an escape mechanism to flee from her relationship
with Jake, whom she really loves but is unable to have sex with her. The one time in
which she does morally assert herself is in disposing of
Romero.

Thus morality is an important theme as characters
reject conventional forms of morality and build their own system of morals, justifying
them as they go, for their own reasons. Such morals have a lot to say about the
characters in this book and the kind of desperate, detached and selfish lives they
lead.