Luke: Obi-Wan.
[Obi-Wan's spirit approaches Luke]
Luke: Ben! Why didn't you tell me? You told me that Darth Vader betrayed and murdered my father.
Obi-Wan: Your father... was seduced by the Dark Side of the Force. He ceased to be the Jedi Anakin Skywalker and "became" Darth Vader. When that happened, the good man who was your father was destroyed. So, what I told was an alternative fact.
Luke: What the **** did you just say?
Obi-Wan: Luke, you're going to find that many of the truths we cling to depend not on facts, but with alternative facts. If reality doesn't fit your narrative, change the facts. Anakin was a good friend. When I first met him, your father was already a great pilot in stop things called pods. But I was amazed how strongly the Force was with him. I took it upon myself to train him as a Jedi. I thought that I could instruct him just as well as Yoda. I was wrong.
Luke: There is still good in him.