This is a throw away line we use often here.
"I know these streets like the back of my hand"
"I know him as well as I know the back of my hand"
I don't make a habit of studying the back of my hand. I kind of "know" it but don't really study it that closely on a regular basis. If you asked me how many spots and freckles on had on there I couldn't tell you. :)
My point is... can we ever really know something or someone as well as we claim to?
My feeling is that once we "think" we know something - or someone - what or who that something/someone is, tends to become "invisible" and we forget to study it or keep on "knowing" it [or them] with any depth or further interest.
You see - we humans tend assimilate just enough information to get our own desires met and once we've done that, we can become somewhat absent-minded about any further need to keep on learning and knowing because we believe in our core we don't need to - we think we already "know"!
People grow and change...endlessly. Things we once assumed were just so become either more so or less so... often. What we once believed can no longer be believed... more often than we think. What we once thought was true is sometimes proved otherwise... or not - which can surprise us too.
Life is a never ending journey of discovery and process. Once we make the assumption that we "know" something in its entirety, we are doomed to be forever wrong in that assumption.
And that is part of the fun of life and living!
To be proved wrong occasionally, is much, much more worth it than being proved right time and again! Being proved wrong forces us to re-evaluate and re-invigorate our processes of discovery. It makes us less sure of ourselves and thus less inclined toward arrogance and pride.
Being proved right can simply make us all the more certain we indeed know the back of our hand! :)