This is an idea coined by Robin Sloan in his book Mr Penumbra’s 24-Hour Bookstore. The idea is to write a book, the Codex Vitae or Book of Life, that represents everything I have learned in my life. If I lost all of my memories somehow, this could be something that helped me remember who I am.
I started my own Codex in 2012. It began as a public Github repository so that I could track (and be accountable) to my own beliefs. As of the end of 2018, the “source of truth” is now here on this site: My beliefs.