Excerpt from a letter by Richard Feynman, mensch:

I judge from your letter that in Venezuela you are teased badly if you are a professor and say you don't know or are not sure. I am glad that I am not so teased because I am sure of nothing, and find myself having to say "I don't know" very often. After all, I was born not knowing and have only had a little time to change that here and there. It is fun to find things you thought you knew, and then to discover you didn't really understand it after all.