I kinda like the statement in my sig. I read it attributed to Shakespeare, although I honestly don't know exactly where it comes from. I realized later that Socrates famously said something similar. “The only true wisdom is in knowing you know nothing.”
Of course, you can't take this too literally. Obviously we know many things today that could not have been known in Socrates' time. They didn't know what the sun was, or what it was made of, or how far away it was, or that the earth spins on its axis and this is why the sun and the moon rise and set every day. Or what the stars and planets are.
But the way I interpret it is to never be too sure that you have it all figured out. Always question your own thinking and stay humble. Ask yourself "but what if I'm wrong?". Are you able to discard a previous position and admit that you were wrong if new evidence becomes available? Or do you look for evidence to confirm what you already believe and ignore any evidence to the contrary?
Do you start with a conclusion and look for evidence to support that conclusion, or do you follow the evidence wherever it may lead, even if it leads somewhere you wish it didn't?