"The One Mind alone is the Buddha, and there is no distinction between the Buddha and sentient things, but that sentient beings are attached to forms and so seek externally for Buddhahood. By their very seeking they lose it, for that is using the Buddha to seek for the Buddha and using mind to grasp Mind. Even though they do their utmost for a full aeon, they will not be able to attain it. They do not know that, if they put a stop to conceptual thought and forget their anxiety, the Buddha will appear before them, for this Mind is the Buddha and the Buddha is all living beings. It is not the less for being manifested in ordinary beings, nor is it greater for being manifest in the Buddhas."
-Huangbo Xiyun (China, d. 850 CE)
Always asking questions, getting answers, then questioning the answer. No wonder the Buddha made the observation that living means struggling. We get something handed to us on a platter, then somehow decide that it isn't what we really wanted, or it is what we wanted, but that was then, and now we've changed our minds.
Maybe changing our "mind" isn't really an accurate statement. Maybe it's semantics, but maybe it's just thinking that changes, not really Mind. Not in the sense that Huangbo uses the word anyway. You could use “Buddha,” “Nirvana,” “Reality,” “Dharmakaya,” “Tathagatagarbha,” or any number of other words for the unchanging, the “not-green/not-yellow”, as Huangbo would say. All those words kinda point to what they point to, but there’s really nothing to be pointed to, unless you want to point at every microcosm in the billions of chiliocosms. Water doesn’t need the fish to point to water, lack of fingers notwithstanding. Reality doesn’t need us to point to it, as if it were somehow separate from us. It’s not Reality +1 on the Great Cosmic Guest List. “It” isn’t missing us, we’re not missing “It.’.
Since the Buddha first turned the Wheel of the Dharma, nobody has really had anything to add, it's all just been rephrasing, using those provisional, conditional words, and doing our best to say something that will help. And just as there is nothing to add to you or me. That’s not to say there aren’t a few things we could eliminate, but fundamentally, we are “there”, we’ve already got “it,” already are “it” for that matter. You can use whatever “noun” you want between the quotation marks, none of them accurately describe “it.”
ZM Seung Sahn would say, "...become enlightened, save all beings."
“Buddha-Nature” means you're already enlightened, or at least have the essence of “Nature of Enlightenment.” All you have to do is act like it. So go save all beings and become enlightened. Become enlightened and save all beings. Either way works. Enlightened action is saving all beings, so there's only one step. Easy. That's your Buddha-Nature, your True Nature. He and others have handed Truth to us, right there on the platter.
That's all the practice is about: helping someone else get through their struggles. Why we do it, how we do it, that's all incidental.
Where is Nirvana? Stand up. Take one pace to the left. Look at where you had just been. Now step back into that spot. Done.