“Sitting in Zazen is not learning Zen meditation. It is the great peaceful and joyful gate of Dharma. It is untainted practice-and-experience.”
– Dogen, Shobogenzo Chapter 58 (Zazengi)

The phrase "it is untainted practice-and-experience" alludes to the conversation between the Sixth Patriarch, Dajian Huineng, and his disciple Nanyue Huairang, to which Dôgen often makes reference:

Huineng asked his student Nanyue "Where do you come from?" when the latter first arrived.

Nanyue said, "I come from the National Teacher An on Mt. Song."

Huineng said, "What is it that comes like this?"

Nanyue was without means to answer. After eight years of consideration, he announced to the Patriarch, "I've understood what you put to me when I first came: 'What is it that comes like this?'"

Thereupon, Huineng asked, "How do you understand it?"

Nanyue replied, "To explain anything would miss the mark."

Huineng said, "Then should one engage in practice-realization or not?"

Nanyue replied, "It is not that there is no practice and realization, only that they cannot be defiled."

Huineng approved, saying, "Just this non-defiling is what all the buddhas keep in mind. You're like this, I'm like this, and all the patriarchs of the Western Heavens [i.e., India] are like this."

The teacher Dogen said: Huineng and Nanyue have spoken like this. Today, how can I not say something? Tell me, great assembly, do you want to understand this clearly? The highest fruit of arhat practice is the new attainment of extinction through discernment. Ajnatakaundinya was verified in his attainment of patience with non-arising. At just such a time, again, how is it?

After a pause, Dogen said: We must smile at the beginning of this story about that fellow Nanyue Huairang. Upon exerting his power, he could express eighty or ninety percent.
- Dharma Hall Discourse No. 374 (Eihei Koroku)

The teacher Dogen said: Do you want to clearly understand the meaning of this? If the sixth ancestor were to ask, “Where are you from?” I would say on behalf of Nanyue: For a long while I have yearned for the atmosphere of the master’s virtue. Arriving here to humbly make prostrations, I cannot bear how deeply moved I feel.

Suppose the sixth ancestor also asked, “What is this that thus comes?” On behalf of Nanyue, facing the sixth ancestor, I would bow and lower my head with hands clasped in shashu position and say: This morning in late spring it is fairly warm, and I humbly wish the venerable master ten thousand joys in your activities.

Suppose someone asked what was the meaning of Nanyue’s statement “To explain anything would miss the mark.” I would simply say to him: Even though the reeds are young and green, these spring days the sunlight remains later, and I would like to build a grass hut.” (The word translated as “reeds” refers to a kind of grass used for building thatched roofs. A grass hut is a metaphor for the space of practice.)

Suppose I was asked, “What did the sixth ancestor mean when he said, ‘Just this nondefilement is exactly what the buddhas protect and care for. You are thus, I am thus, and the ancestors in India also are thus’?” Then I would like to say: A blue lotus blossom opens toward the sun. (A blue lotus is utpala in Sanskrit, one of the four kinds of lotus flowers in India. The Mahaparinirvana Sutra, chap. 24, says that it represents diligence.)
- Dharma Hall Discourse No. 490 (Eihei Koroku)

Having fully cooked all his stuff, this Nanyue
Played with blowing winds and saw arising clouds,
Tasted the dragon’s scream, and loved the dragon’s howl.
Single-mindedly striving,
For eight years refining gold,
Dropping body, dropping body,
Did he clearly get it or not?
What is this thus come and thus appeared?
The mind before your father and mother were born.
Although directly attaining the wonder right now,
Vipasyin Buddha maintained this mind before.
- Verse 59, Master Dogen’s Verses Praising Ancient Koans