…[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