Nothingness

But completely experiencing the nature of the mind involves complete openness, or complete nothingness; when you really experience the nature of the mind, there is utter stillness with no observer observing anything, no experience, thought or label. (Diamond Heart Book 2, pg 17)

Nothing exists without that nothingness. Anything that exists needs some sort of space in which to exist. So, not only is it the ground of all experience, it is the ground of everything. It is seen as the basic nature of reality, as the deepest nature of reality. When everything quiets down, ultimately there is nothing. It is not that someone is looking, and can't find anything; in that process of looking, you start looking for yourself, and you don't find it. And finally, there is nothing. This doesn't mean that the physical body doesn't exist. There is simply no entity there producing, proceeding, or organizing these impressions, beyond the impressions themselves. There are simply the impressions that come and go; they come from nowhere and go nowhere. And then all impressions can cease, revealing complete emptiness. This is taken to be the most basic nature of reality, the ground of all existence. We call it space here because this experience is more like space. (Diamond Heart Book 2, pg 17)

In some sense, nothingness is the unstructured and unimpeded nature of pure presence. We experience this nothingness psychologically as total openness. We feel we are completely open, with no limitations, boundaries, restrictions, differentiations, or any recognizable features. We are the total openness of true nature, open for any arising and perception. The openness is both phenomenological and psychological. We feel our presence phenomenologically as the openness, nothingness so total that it does not impede anything. (Inner Journey Home, pg 306)