Obviously

 I know that I don't know.

That statement deserves some attention because it seems to be important, something I trust to guide my choices and keep me out of trouble. 

How can I know and not know? Isn't that a contradiction, logic confounded in paradox like Gödel's Incompleteness Theorems? No, don't get lost down that rabbit hole. Is it like quantum mechanics where particles are also probability waves; is knowing actually different things at the same time? No, that's not right either. 

It's a semantic trick. The word know can mean different things in different contexts. It could mean that I am sure what I know could not be otherwise in this universe. Or it can mean I have direct experience, or I have information from a trusted source, or I have a conviction based on the agreement of instincts, appetites, aversions, sensation, perception, memory, emotion, imagination, empathy, values, conscience, reason, etc. Or what I claim to know could be a simplification, a metaphor, a story, a fib or a compromise used to motivate and get things done in a world that is too complex to understand. 

So, I'm saying that my various mental faculties agree that the information I have is fallible and incomplete, and it could not be otherwise in this universe; and I've been pretending I know just to get things done. And this is my honest confession of ignorance.

I know that I don't know.

Obviously.