May be everyone knows this but I feel it is one of those things that are present in the subconscious, not obvious, just like every creature knows that jumping from a height could either injure or kill it.
Many applications on Facebook these days are quizzes which are supposed to reveal how much one person knows another person. Essentially it is fact-based. Knowing facts about a person is a major 'fact'or in knowing a person. Agreed! However, I realized that you could just know a person or feel that you can know a person without actually being able to quote their favorite color, dish, brand or pastime. In fact these may change from time to time.
So if you want to know a person, you should know how their tastes may vary or how they react to events and why. You could perhaps say you know a person well if you can think what the other person is thinking. You'll know the chain of thoughts. Maybe why he/she thought of that thought even! Everything. And yet, you may not be able to tell something specific like the other person's dog's name.
With my new found discovery (having brought it from subconscious to main working sheet of the mind) I plan to set out trying to know people. My emphasis will be on extrapolation of person's behavior to judge (Oh everyone judges everyone at every instant of time!<--- I just did once again!) the person and predict the person's reactions at another time. The joy you get when you realize that you can almost predict what a person would do in a situation and then see that the prediction becomes true is un-word-able! :)
I have some friends who can tell what my reaction would be and why. We often don't even need words to communicate with each other. (This comes to the fore when you're playing games like Dumb Charades and Taboo.) It is mutual. I am now discovering/adding more people in that list.
PS: Maybe the idea behind knowing facts is to know what happened in the past and what was the response of that person. That knowledge may help in predictions. Any which way, it is not just the facts...
