心の月を Kokoro no tsuki wo
ながむるも Nagamuru mo
闇に迷ひて Yami ni mayoi te
色にめでけり Iro ni medekeri
Although seeing the moon of the mind,
in the great sky,
being deluded in the darkness,
I praise it for its shape and color.
"The moon of the mind" (Kokoro no tsuki) is the key phrase of this waka. What is "the mind" here? Dōgen quotes Zen Master Panshan Baoji's saying in Shobogenzo Tsuki (the Moon):
"The mind-moon is alone and completely round. Its light swallows the myriad phenomenal things. The light does not illuminate objects. Nor do any objects exist. Light and objects simultaneously vanish. Then What is this?"
The moon light swallows all things and there is nothing outside of the moon. This is what "alone and completely round" means.
In Shobogenzo Sokushinzebutsu (The Mind Is Itself Buddha), Dōgen said:
"The mind that has been authentically transmitted is 'one mind is all dharmas; all dharmas are one mind.' For this reason an ancient said, 'When we understand the mind, there is not an inch of soil on the great earth.' We should know that when we understand the mind, the entire sky is struck down and the whole earth is ripped apart."
Oozora (大空) is literally the great sky but it also can mean great "emptiness". In this mind, all things (both subjects and objects) are included. "Great" in this case means absolute, before separation between the subject (sense organs) and object of sense organs. "Iro (色)" is shape and color, but this kanji can also means "form", as in "form and emptiness" in the Heart Sutra. We commonly appreciate the moon in the sky as the object of our eyes. It entertains our mind, and we feel joy or sadness depending upon the condition of our mind. Dōgen says this way of viewing the moon is delusion in darkness.
We can interpret this waka in two ways: one is an admonishment to other people who only see the moon as an object of their mind. Or, we can see the poem as a description of Dōgen's regret for himself, when he did not see the moon including subject and object. Most commentators interpret this waka with this meaning. The other is to understand this poem as a reflection of Dōgen's own appreciation of the beautiful moon as an object of his mind, as many waka poets compose their poems. In this case, "being deluded in the darkness," is not really negative. It is like returning to delusion from great realization, in the same way as he discusses in Shobogenzo Daigo (Great Realization). I think Dogen's last waka composed a few weeks before his death shows this:
I [was not sure] if I could expect to see the autumn again,
[Gratefully I can see] the full moon of this night,
How is it possible for me to sleep?
Dōgen expresses the same meaning in another waka:
Seeing flowers in spring,
crimson leaves in autumn,
and white snow in winter,
I am regretful
that I have appreciated them as the objects
[that entertain my feeling].
In this waka, he simply changed "mind-moon" to the beauty of spring, autumn, and winter. Same as the above waka, I don't feel he was seriously regretful for appreciating the beauty of the seasons as objects.