Yoda: You see, Luke, all about Chewbacca I know. Good friends we were, before you met him...very good friends indeed.
In ESB while Luke is training:
Yoda: The Force, magical it is not. Rather, it is these tiny organisms...
In ROTJ:
Luke: Is Darth Vader my father?
Yoda: Your father he once was. But once he became Darth Vader, he separated himself from himself he did. Okay, confusing it sounds so let me explain: if he were to die today and still be bad, he will appear as he does now. But if he were to die good again, he would appear as he did BEFORE Darth Vader he became.
