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If pushed, I'd say the core of any Zen practice is striving for insight into the fundamentally brutal question of "Who am I?" Some practices try to sugar coat this by saying it's "What am I?" or "What is the nature of my mind?" or some such, but that's trying to elevate the question to some impersonal observation. It's saying "Draw me a map of the world" instead of "Show me the crappy place where you live."
The question, if asked correctly, is not impersonal. It's about as personal as you can get. To begin with, it's forcing you to see who you really are, not who you want to be. You're selfish and vain and timid and afraid and angry and miserable because you don't like being that. You're frustrated because of your job, or your income, or your relationships, or lack of all three. You're not as smart, or pretty, or talented as those people you see out there. That is who you are: one big mess.
So you want to be someone else. Here's where Zen comes in. Wanting to be someone else is making you miserable. People don't "change". They become. People never change who they are, they simply stop pretending to be someone else. This is called your "Buddha Nature."
Me, I'm a generous, easy-going guy who doesn't make many friends because I get bored with what passes for a life with most people. I have a low tolerance for bullies or bullshit and a smart mouth at times and I love to talk. That's who I've always been since I can remember. Anything beyond that is me trying to be someone else. For instance, I thought for a while I could be a Zen Master and spend my days contemplating the Emptiness of it all. I think that ended when I left Korea. My bullshit meter couldn't be ignored anymore, and believe me, a whole lot of what you think is Zen Buddhism is as much bullshit as any other practice. You know how the koan says, "If you meet Buddha on the road to enlightenment, kill him."? I didn't kill him, I just gave him a wedgie and laughed as I replace zazen with just sitting quietly, doing nothing. When the moment called for sitting quietly, doing nothing.
So for meditation, "Who am I?" can be a very personal question. Followed by "What the hell do I think I'm doing?" The answer might surprise you.
Thanks for listening.