I think I'm a pretty smart guy. Let me qualify that.
Since High school I've kept a certain saying as a guide. I got it from Socrates, by way of Plato, by way of my Humanities teacher Mr. Wiltgen. It is this: "The only thing that I know for certain, is that I know nothing."
This is a wonderful theoretical foundation for any person who wants to keep his mind open. However, it is not a practical philosophy for everyday use.
For practical purposes I know many things. I know better than to stick my finger in a light socket. I am familiar with the use of language. I know how to put one foot in front of the other and walk down the street.
Still, for all that I know in the practical world I must constantly remind myself that " The only thing that I know for certain, is that I know nothing."