The primary concept underlying Dōgen's Zen practice is "oneness of practice-enlightenment" (修證一如 shushō-ittō / shushō-ichinyo).
For Dōgen, the practice of zazen and the experience of enlightenment were one and the same. This point was succinctly stressed by Dōgen in the Fukan Zazengi, the first text that he composed upon his return to Japan from China:
To practice the Way singleheartedly is, in itself, enlightenment. There is no gap between practice and enlightenment or zazen and daily life.
Earlier in the same text, the basis of this identity is explained in more detail:
Zazen is not "step-by-step meditation". Rather it is simply the easy and pleasant practice of a Buddha, the realization of the Buddha's Wisdom. The Truth appears, there being no delusion. If you understand this, you are completely free, like a dragon that has obtained water or a tiger that reclines on a mountain. The supreme Law will then appear of itself, and you will be free of weariness and confusion.
The "oneness of practice-enlightenment" was also a point stressed in the Bendōwa (弁道話 "A Talk on the Endeavor of the Path") of 1231:
Thinking that practice and enlightenment are not one is no more than a view that is outside the Way. In buddha-dharma [i.e. Buddhism], practice and enlightenment are one and the same. Because it is the practice of enlightenment, a beginner's wholehearted practice of the Way is exactly the totality of original enlightenment. For this reason, in conveying the essential attitude for practice, it is taught not to wait for enlightenment outside practice.
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