When asked his name in the street as a young boy, Huineng replied, "My name is Buddha Nature."
Later, Dogen reframed this as impermance itself is Buddha Nature. This pulls the rug out of any fixed view of Buddha Nature. It's not something you can have or even be called. It brings up doubt.
Joan Stambaugh tells us, "To begin to understand impermance one must first of all doubt."