What you thought and believed

"A general limitation of the human mind is its imperfect ability to reconstruct past states of knowledge, or beliefs that have changed." Says Daniel Kahneman in Thinking, Fast and Slow. "Once you adopt a new view of the world (or of any part of it), you immediately lose much of your ability to recall what you used to believe before your mind changed."

Writing down your past states of knowledge and beliefs—keeping memories—is helpful because they will change.