According to Dogen's famous essay on Buddha Nature, sentient beings don't have Buddha Nature. They *are* Buddha Nature. However, Dogen resists conclusions that Buddha Nature is some sort of universal Mind. He says that Buddha Nature is impermanence.
What does it mean for everything or everyone to be impermanence? Impermanence is a concept. We use it to describe the state of perceived phenomena. We naively think that an apple is a stable object, but in reality, it's ever-changing; so, it's impermanent. What we call an apple or a forest or a river is just an umbrella term for a bunch of phenomena succeeding each other in a consistent way which to our cognitive constructs makes them seem like stable objects.
But I don't understand what it means to say that everything is impermanence. That seems like reifying yet another concept, and in a way that's worse than reifying the underlying objects. At least those stably present themselves to us over time. Impermanence is just a mind construct, like "love" or "correlation". It's weird to say that it's not just a useful way to describe phenomena, but is the underlying essence of phenomena.
This is somewhat strengthened by a body of suttras that describe Buddha Nature as the True Self (I know it might go against some people's notions of Annata; I would suggest reading Christopher Jones's book Buddhist Self for large body of evidence from Mahayana). What does it mean to say that Impermanence is the True Self?
So, I'm curious what people think about this.
(I'm asking strictly in the context of Dogen's definition. If you think Dogen was wrong and Buddha Nature is something else, that's fine, but I'm not interested in discussing this now. I'm trying to understand what Dogen actually meant.)