Shobogenzo was written by Master Dogen in the thirteenth century. I think that reading Shobogenzo is the best way to come to an exact understanding of Buddhist theory, because Master Dogen was outstanding in his ability to understand and explain Buddhism rationally.
Of course, Master Dogen did not depart from traditional Buddhist thought. But at the same time, his thought as expressed in Shobogenzo follows his own unique method of presentation. If we understand what this method is, Shobogenzo is not so difficult to read. But unless we understand his method of thinking, it is completely impossible for us to understand what Master Dogen is trying to say in Shobogenzo.
Buddhists revere Buddha, Dharma and Samgha. Buddha means Gautama Buddha. Samgha means those people who pursue Gautama Buddha’s truth. Dharma means reality. Master Dogen’s unique method of thought is his way to explain what Dharma is.