[W]e keep up this constant vigil and strive to carry on re-creating and re-defining a “self” and making sure this “self” is “good” according to whatever standard of “good” we have learned. Or we fret endlessly over the parts of this “self” that don’t fit that definition of “good.” We waste a lot of energy on that.It’s this “self” that we have created that wants tranquility or peace or enlightenment or to be free from anger or whatever. But it can’t have those things. Because it wants to have them, to possess them, and that stuff can’t be possessed. Nothing the self thinks it has, thinks it possesses, including anger, fear, doubt, whatever it wants to be free from, none of those things can be possessed either. They all come and go. Even your anger isn’t really yours. So freedom from anger can’t be yours either. It’s just a losing strategy from the start. There’s no point in trying to make any of that happen.
…[H]ere is the idea that if you understand just one thing completely, you understand everything. This might be one of Dogen’s greatest insights. It informs everything about how we do zazen practice in the Soto style. It’s why we don’t use mantras or concentrate on koans or count our breaths or anything else. It’s the philosophical rationale for the practice of shikantaza or “just sitting.” If you understand just one thing thoroughly, you understand everything. For your object of study, why not choose the closest thing there is, the easiest one to look at, the one you’re already most familiar with? Why not look at yourself?
Source: The One Thing | Hardcore Zen