That we don't really know. I mean that about everything. It's drilled into our heads that we have to speak with certainty and confidence about things, especially if we are "expert" in these things, but the truth is that we don't know.

That's too vague. Here's an example:

I've been doing Brazilian jiujitsu for 13 years. I've been teaching it for 8 or 9. I'm a good teacher and I often speak with authority about things that I feel like I know well. Two things always happen;

1. I always find someone more knowledgeable that explains something to me that I already thought I knew. I then realize that my understanding was juvenile at best and that I now know more.

2. If I look back at my assertions from a year ago, I'm amazed at how primitive they are but at the time I was convinced that they were the truth. A year from now, I'll be writing the same thing about the assertions I'm making now.

I beleive in being confident in my beliefs. I also think this should be tempered by a strong sense of self deprecating humor about how I really don't know anything.

I think people don't want to admit that they just don't know.