Logically, I totally agree with this -- it's not an uncommon belief, after all. I've heard many variations of it, but not many focus as much on how we know ourselves as part of the equation. Sometimes I react well (or don't react, as Tolle often stresses -- rather than react, he suggests that we simply accept and absorb) and am able to remain 'myself' regardless of the emotions and circumstances around me. Other times...well, I fall apart ;)
How about you? When do you feel you've known yourself best?