"Moreover, I found out one memorable day how little we really know even about things we consider to be solid facts--in this case I had been bloviating about some matter of chemistry or other, had casually mentioned the definition of a "mole", and was brought up short with the flat question, "How do you _know_ that?" "Uh, I read it," is not a good answer. To my credit I was able to figure out a way experimentally to determine the number of molecules in a mole, but then _that's_ based on certain other assumptions that I knew even less about. And so on down the line. We take much more on faith than most people care to think about."