I just finished reading a short chapter titled "The Dharma Nature" in Dogen's Shobogenzo. It's chapter 54 in Book 3 of the Nishijima/Cross English translation. ( you can download book 3 here)
"Buddha Nature" is a phrase you come across a lot in Buddhism, but Dogen's the only person I've read so far who talks about Dharma Nature. In the intro to the Nishijima/Cross translation, they explain Dharma Nature as meaning the "essence of the universe", which makes it a pretty big deal. Here's the full intro:
Ho means Dharma, that is the Buddha's teaching, or the Universe itself. Sho means essence, or nature. So hossho means the Dharma-nature, or the essence of the Universe. Needless to say, we are living in the Universe. Therefore what the Universe means is one of the most important philosophical problems in our life. Some people insist that the Universe is something spiritual. Others insist that the Universe is something material. But from the Buddhist standpoint, the Universe is neither spiritual nor material, but something real. It is, however, very difficult to express the Universe as something real using words, because reality usually transcends explanation with words. Master Dogen undertook this difficult task, in order to express the nature of the Universe, in this chapter.
It's one of those chapters you don't hear about so often, but it's a nice one nonetheless. Worth a look if you're interested in that kind of thing.