Emptiness basically means “boundless” or “no-origin.”
Instead of there being things-unto-themselves with innate attributes, beginnings, middles, and ends, there are things-unto-their-not-selves. When we see all the causes and conditions related to something’s state of being, we see that it’s ridiculous to consider it separate from those causes and conditions.
Since those causes and conditions are infinite in time and space, all things are such as well.
We also see that the perception of a thing-unto-itself, is just that: a perception. To keep us from going from one extreme to another, the perception of a thing-unto-its-not-self, is also just a perception. But, it’s a perception that transforms suffering into joy, well-being, tranquility, and equanimity.
Buddha-nature has two connotations. 1) All beings have the potential to Wake Up and become Buddhas, and 2) All beings are Buddhas already, we’re just unaware of it. Another interpretation sets it up as a positive take on emptiness. Some, more essentialist Buddhists, even consider it akin to a kind of Absolute or God Mind, but I’m not ready to go that far with it because that is far too tempting to grasp onto.
So, how do we reconcile emptiness with the view that we’re all Buddhas or that we can all Wake Up? If there’s really no us but a vast net of interdependence, then who is that can Wake Up? Who is it that has that potential?
It’s pretty simple, really. Let’s say we’re hanging out in a pub somewhere. In that room, you’re breathing in what I’m breathing out and vice versa. Because of that, there’s no such thing as my air and your air, and the air quality is the same for both of us. The only difference is how we breathe.
Since emptiness means that this and that are equal and linked through an interdependent relationship, if Awakening is possible or actualized at one spot in this net, then—just like how we share air—it’s possible and present everywhere.
Picture Siddhartha, or whomever, sitting under that tree 2,500 years ago and completely Waking Up. Then, like a stone being cast into a pond, that Awakening rippled through time and space. Of course, those closest to that moment had it easier because the waves were more pronounced.
But, even though they’re gentler now, they’re still present. They’re present in all the teachings and methods we have access to, and present in our ordinary minds as well.
Buddha-nature is that stone, emptiness is the nature of that rock being able to fall into water, and it’s the water’s nature to ripple and flow.
Of course, in this case, the water isn’t a substance, it’s the ongoing and unlimited interaction and interpenetration of all causes and conditions.
All Awakenings are that same Awakening, just at different depths. It’s the awareness of those ripples, and the temporary setting aside of our self-concepts and grasping, so that we can see through our this or that perception to the principle at work in all times and places.
So, the emptiness, Buddha-nature, and mind-only teachings aren’t mutually exclusive. In fact, they depend on each other, since they’re also empty. Without Buddha-nature and mind-only, the emptiness teachings are basically nihilism. Without the emptiness and mind-only teachings, Buddha-nature is akin to some sort of God (Gross). And without the emptiness and Buddha-nature teachings, mind-only is no different than solipsism, or Descartes’ confused, misguided idealism.
And all of those teachings, though later Mahayana additions, can be traced back to the Pali Canon, to the earliest teachings. Mahayanists just took what was already present, and expanded it to a cosmic scale so that it could reach more people, and allow people to practice without joining a monastery.
Then Chan came long and put it all under one roof, when an emphasis on Waking Up in this lifetime.