Principles vs. God

The Bonhoeffer biography is so good...it's one of those books where you wish it just wouldn't end. I came across this the other day:

..."Christians cannot be governed by mere principles. Principles could carry one only so far. At some point every person must hear from God, must know what God was calling him to do, apart from others."

Eric Metaxas in Bonhoeffer: Pastor, Martyr, Prophet, Spy; emphasis mine