Write this down: East of Eden hinges on the word timshel. That's it. That's the entire explanation. As Lee points out, timshel is a Hebrew word meaning thou mayest, and it's in the original story of Genesis when God promises Cain that if he does well, he will be rewarded, and if he doesn't, well, too bad. So it's this whole idea of choice: Cain doesn't have to get all mad and kill Abel—just like Cal doesn't have to ruin his brother's image of their mom. At the same time, there is this underlying worry in the novel that the Cain and Abel story will somehow repeat itself.
Questions About Fate and Free Will
Do we ever see the hand of fate at work in East of Eden?
To what extent is fate a legitimate excuse in the novel?
Why do you think Steinbeck is so big on the idea of free will? What does he say about it in the novel?
Why do you think Steinbeck went with the word timshel instead of consistently translating it to Thou mayest?
Chew on This
The novel portrays choice as a difficult thing to grapple with. Fate is a much easier excuse.
The hand of fate is oddly present in the novel, even though the characters supposedly have free will.