Zazen is not based upon teaching, practice, or realization; instead these three aspects are all contained within it. Measuring realization is based upon some notion of enlightenment—this is not the essence of zazen. Practice is based upon strenuous application—this is not the essence of zazen. Teaching is based upon freeing from evil and cultivating good—this is not the essence of zazen.
From “Zazen Yojinki: Notes on What To Be Aware of in Zazen” by Keizan Jokin (from The Art of Just Sitting, Wisdom Publications, 2004)