Dōgen Zenji, or Eihei Dōgen, (1200-1253) was a Japanese monk credited with restoring the quality of Buddhism in Japan and transmitting Soto (Caodong) and "just sitting" meditation from China to Japan. Beginning as a Tendai monk, Dogen is said to have grown dissatisfied with the one-sidedness of the Tendai teachings and, while remaining deeply influenced by them, sought out teachers of Chan (Zen). His magnum opus is the Kana Shōbōgenzō, usually just referred to as the Shōbōgenzō ("Treasury of the True Dharma Eye"). However, this book was not particularly widely-read, even in Buddhist circles, until the 19th century.
If you are looking for a good introduction to Dogen, check out Shohaku Okumura's "Realizing Genjokoan", a commentary on the "essence" of the Shōbōgenzō:
The "essence" of the Shōbōgenzō is considered to be the short treatise entitled "Genjokoan", a surprisingly difficult term to translate completely, but which we will say means "to answer the question from reality through the practice of our everyday activity". If taken in parts, "genjo" can mean "to manifest" and "koan" can mean "a question posed to us by reality" or "equalizing inequality". Note that "koan" refers both to the equality of all things (ko) and to their uniqueness (an), and that equalizing inequality does NOT mean getting rid of differences - rather, Genjokoan is the intersection of sameness and difference. Reality includes both unity and particularity. Absolute truth embraces relative truth. Okumura asks "how can we actualize both [of these] sides of our lives within [each] action? This is really the basic point of our lives."
The Shōbōgenzō itself is very difficult due to its highly literary and thematic nature, and requires a good deal of familiarity with Buddhist ideas, literary tropes (especially symbolism), and the sayings of previous masters. One of the most important contributions of the Shōbōgenzō is the transition of Chinese ideas into a Japanese context (much like what happened when Buddhism moved from India to China), which often involves modifying or viewing the themes discussed from alternative perspectives than the then-traditional ones.
There are two closely related things to understand about Dogen: (1) that the central question of his spiritual quest and subsequent teaching was "If all things have Buddha-nature and are thus 'originally' enlightened, how come we still have to practice?" and (2) that only practice and living life right here and right now can go beyond conceptualization. To quote Okumura:
Dogen cautions us not to live our lives according to our thinking; he admonishes us rather to just see and to just live. To live in this way means that we just see and experience what we encounter in our lives without saying it is empty, even though it truly is empty. That's it. We don't need to say "This is empty" if it really is empty.
Back to Genjokoan:
Those who have great realization of delusion are buddhas; those who are greatly deluded about realization are sentient beings. Further, there are those who continue realizing beyond realization, who are in delusion throughout delusion.
This is a very typical line from him, reminiscent of his take on the HeartSutra (which is also very representative of point number two):
"form is emptiness and emptiness is form." Form is nothing but form, emptiness is nothing but emptiness - one hundred blades of grass, ten thousand things.
1) As all things are buddha-dharma, there are delusion, realization, practice, birth and death, buddhas and sentient beings.
2) As myriad things are without an abiding self, there is no delusion, no realization, no buddha, no sentient being, no birth and death.
3) The buddha way, in essence, is leaping clear of abundance and lack; thus there are birth and death, delusion and realization, sentient beings and buddhas. Yet blossoms fall, though we love them; and weeds spread, though we hate them.
1 - the "first turning", 2 - the Heart Sutra, 3 - Dogen's own approach
Buddha's toilet gatha, Nishijima translation:
You were never stricken by poverty,
Nor have you lost wealth and nobility.
Only in order to pursue the truth, you have left home.
You will be able to endure the hardship.