The single most important idea in Chinese Buddhism, and hence in Zen, is Buddha nature. And it has a historical context.
The three strands that make up Chinese Buddhism in its initial period are Nagarjuna, Yogacara, and a clutch of sutras, which don’t appear to have specific schools attached to them, but which centre around the theme of Buddha nature.
And we can see in a way these three aspects are sequential in that Yogacara is correcting an erroneous reading of Nagarjuna, and some themes in Yogacara are then developed in Buddha Nature thinking.
Nagarjuna’s destruction of all our conceptual apparatus in the affirmation of emptiness was meant to clear the ground for us to live a life that is more connected, more within the world, and more alive. The problem is that from the get go, many people misinterpreted Nagarjuna in a nihilistic way, because the positive aspect was left implicit.
So Yogacara, which comes a little bit after Nagarjuna, is a corrective to that, in several ways. It creates a very elaborate Buddhist theory of consciousness, and instead of emptiness it talks about suchness. It says we can be in three states. We can be in an illusory state where we’re just seeing the world in a conceptual way. We can see the world as dependently originated. And we can see the world as thusness, the third stage. Seeing the world as dependent, particularly during meditation, is a bridge to thusness. But Yogacara doesn’t really detail, possibly for the same reasons as Nagarjuna, the positive aspect of the doctrine, what thusness is, because explaining something leads you to the errors of conceptualising and comparing it, thinking there is a standard that you’ve got to reach and so on. So Buddha nature then comes along and fleshes out this idea of thusness in a series of positive attributes.
The problem that we have then, from our Zen perspective, is that these three tendencies, which are put together in texts like the Lankavatara Sutra, which then feed into the creation of the indigenous Chinese schools of Tiantai and Huayan, which leads to Zen in due course are, apart from Nagarjuna, largely forgotten. The Yogacara school disappears due to a disagreement about the extent and universality of Buddha nature, and the Buddha Nature orientation was never a school in the first place.
And so, ironically, the person who these tendencies were meant to fix, or fix an erroneous interpretation of, Nagarjuna, is the guy that remains most visible in the Zen tradition. And this makes it very difficult, I think, for us to understand the significance of Buddha nature thinking, creating a sort of bemused embarrassment, of thinking it’s quaint and Chinese rather than grappling with something absolutely essential for us and which deserves our sustained attention.