Zazen is not thinking good,
it is not thinking bad.
It is not mental activity of any kind;
it is not contemplation or reflection.
Have no intention to become a Buddha.
You must cast off your sitting. . . 

As you sit, meditating deeply and immovably,
think of not-thinking. What is thinking of not-thinking?  
This, in and of itself, is the art of zazen.

Zazen is not learning Zen.
It is the Dharma Gate of great repose and bliss.
It is undefiled practice-realization

—Dogen (S, "Zazengi")