In the book of Genesis, in the Old Testament of the Bible, we are told the story of Cain and Abel, the first two sons of Adam and Eve. Cain murders his brother because he is envious of him, and—when asked by God where his brother is—replies, "I don't know. Am I my brother's keeper?" This is a key moral question that is at the heart of much of Steinbeck's writing.

In East of Eden, Lee calls the Cain and Abel story the "symbol story of the human soul" because the story epitomizes the struggle between selfishness and altruism that everyone experiences, and which arguably defines every moral dilemma we might face. We can all relate to the idea of having to choose between what is in our own self-interest and what is in the wider interests of others. A cursory reading of Steinbeck's writing strongly suggests that he thinks we should act more often in the interests of others and that we should all be our brother's keeper.