When I was a kid, especially when I was a teenager, I was pretty darn sure that I was right all the time. On even the flimsiest of evidence, I was very quick to decide that I knew the way things were. When I had an idea about the way things were, it was nearly impossible to convince me that I was wrong--even when I was wrong.
Dad isn't like me. When I was a kid, he was right all the time. After he'd proved me wrong for the umpteenth time, in frustration, I asked, "How can you be right all the time?!!?" Dad responded, "I'm not right all the time. I just don't tell you things unless I'm sure that I am right."
Dad's words illustrate a key principle. He didn't know everything. But he knew when he knew. He didn't make a truth claim unless he was sure he was right.
In my work, I conduct quite a few job interviews of college students. They are so eager to be right, to answer every question, that sometimes they guess. They don't know. They probably know that the don't know. I sometimes interrupt them. "Don't guess. If you don't know, that's alright. Just say you don't know." I want them to be like Dad.
What does it mean to be like Dad, to know what you know? It means to operate with certainty when you have high confidence in what you know. It means to operate with humility and caution when you have low confidence in what you know. Most important: The wisdom to know the difference.