One of the early scholarly texts (in English) on Dōgen's teaching was Carl Bielefeldt's Dōgen's Manuals of Zen Meditation (1988). I read it soon after publication and found it eye-popping!
The power of it's message has not faded with age. Indeed, I recommend it for any serious Zen student.
Now, Bielefeldt is a scholar and not a Zen teacher. However, he was an early student of Suzuki Rōshi and the old teacher encouraged the young Carl to become a Buddhist scholar. Nevertheless, his interpretations of Dōgen should be weighed with that consideration - he's a Buddhist scholar.
Yet, when he compares what Dōgen writes with one of Dahui's letters, well, that seriously bursts a bunch of Sōtō orthodoxy bubbles (that maintains that the view of these two great teachers were dramatically different). And that's what he does in the passage below.
First, I'll share his translation of Dōgen presenting the mu kōan from The Guidelines for Practicing the Way (Gakudo yojinshu) #8, then Dōgens comments (which for those of you familiar with the kōan process will probably seem like checking questions). And then Bielefeldt adds his thoughts:
A monk asked Zhàozhōu, "Does a dog have the Buddha nature?"
Zhàozhōu answered, "No" (Mu).
Can you gauge this word, "no" [muji]? Can you contain it? There is no place at all to grasp its nose. Please try letting it go. Let it go and just look. What about your body and mind? What about your conduct? What about life and death? What about the dharma of the Buddha, the dharma of the world? What about, after all, the mountains, rivers and great earth, the humans, animals, houses and dwellings?
If you keep looking and looking, the two attributes of motion and rest will naturally become perfectly clear, without arising. Yet, when these do not arise, this does not mean that one becomes rigidly fixed. There are many people who fail to realize this and are confused about it. Students, you only attain it when you are still half way; when you are all the way, do not stop. Press on, press on!
Bielefeldt comments: Surely there is nothing here that could not have been said by Dahui the chief advocate of Zhàozhōu's mu; the entire passage reads quite like one of Dahui's letters. The mystery of the T'ang master's cryptic "mu" becomes a symbol of the larger mystery of Zen and, indeed, of things in general.
If we just keep looking (kan rai kan kyo) at this mystery long enough, we will naturally achieve the calm clarity beyond motion and rest that we have earlier seen to be the deeper meaning of the regulation of body and mind in meditation. To put this practice of looking to work in zazen would seem to require nothing more than a cushion.