Beings are impermanent. Being is permanent. If we accept as conventional Buddhist wisdom that beings are without exception of the nature of impermanence (and its corollary, devoid of the transmigrating soul escape hatch), then we must accept that something (not some thing), some quality of existence of equal weight with impermanence, is of the nature of permanency. We have eliminated all beings from that consideration, so permanency cannot be conflated with a being as entity. It must be the state before the big bang, the empty kalpa. Which are simply big ideas we throw against the wall, hoping something sticks.  

Some would argue that we cannot really know that being is permanent, or whether it too arises with a big bang, for example. In which case, nothing would be before the bang. Which would raise the question, then, how did it come into being from non-being? It would constitute a reverse tautology, a logical self-contradiction, would it not, for what is, after all, a being to argue the reality of non-being. It would amount to, as Matsuoka Roshi mentions in passing, analyzing oneself out of existence. Or positing a notion of time in which for the time being, there is being; but at another time-being, there is, or was, no being. Or, further, to argue that there is a time in which there is no time.

If non-being is not conceivable, then being cannot be impermanent. What is present in being is ever-changing, so the form of being is itself ever-changing, or impermanent. Along with the other four skandhas of clinging.

The many may be regarded as the forms of being, or all beings, while the one is being itself. In the Tao te Ching the question is posed:
The many return to the one; to what does the one return?
It is tempting to say, simply, that the one returns to the many, but this would be far too facile a dodge, and would result in an endless vacillation, rather than resolving the koan.

Here, we might usefully substitute beings for the many; and being for the one. It is not necessary to then ascribe a being to the totality of being. That would be to assert the oneness as superior, in the sense of hierarchy, to the manyness. In Zen, we say that the most you can say is not-two. This is not the same as asserting one, i.e. the one-sided view of unity.

Grasping the “underlying unity” may be regarded as a legitimate kind of insight or  enlightenment (a phrase that Matsuoka Roshi used to describe Zen experience at 180º, halfway round the circle, in his “360º Zen”). However, this does not assert the dominance of the one over the many. Rather, as it is only halfway round the circle to 360º, it indicates the opposite — that this unitary view, while having some validity, is explicitly incomplete.

In the concrete truth of Buddhism, both the one and the many are subsumed under the broader embrace of Master Dogen’s backward step. They may be regarded as conceptual doppelgangers — mutually defining aspects of the discriminating mind, flip sides of the same conceptual coin. Each represents a general principle that has been extracted from one’s many particular case experiences (to paraphrase Buckminster Fuller’s cogent definition of human intelligence). As a polar pair that “always and only co-exist” (Bucky again), not only do they define each other — they enjoy no independent definition, or meaningful existence, outside of the dual dyad. They represent neither true opposites, nor an identity. The one does not equal the many, and vice-versa. They cannot be separated to begin with, so they cannot be placed in a formula, on opposite sides of an equal sign. All this analysis is valuable only if it frees us from the tyrany of language.

The discriminating mind, and its primordial invention, language, both being necessarily dualistic, its pairs of opposite terms and concepts have meaning only by virtue of their relationship. Which is complementary, not competitive. Each completes the other, in Zen’s conceptual analysis of the concrete. Yet they do not exist, as such, in concrete reality. It is not that there is no reality to the one, or to its complement, the many, but neither may be taken as a complete description of reality. Even when taken together, they fall short of comprehending reality; so much less when regarded as separate concepts. Thus, Buddhism "leaps aside" from them.

Buddha’s first sermon ends with the expression, “…now there is no more becoming.” This becoming can then be set up against being, as the classic Buddhist view of the process of all beings arising, abiding, changing, and decaying in the cycle of impermanence and rebirth. All forms of being are constantly becoming.

That some ordinary human being, whom we ordinarily refer to as Buddha, or Shakyamuni, could state that in the present, eternal moment, for him, there is no more becoming (if we are to believe him), implies that there is something that has realized cessation. And that something, while still existent, no longer exists in the "cycle of creation of suffering for ourselves or for others” (Metta Sutta). This something can not be considered a thing, nor a being of any sort, as all such beings are fully enmeshed the cycle of becoming, by definition. So what it is that is making this statement cannot be a being in the sense of an individuated self, but must be being itself, though manifesting, temporarily, as a human being.

This former undefined being may be referred to as the dharmakaya, or essence body. The latter, nirmanakaya, or body of transformation. The body that is ever-becoming, that carnal body through which buddhas appear in the world, through realization of the dharmakaya, that is not "their" body. This is not as circular as it sounds, in that the realization of the dharmakaya is totally dependent upon the birth of the nirmanakaya into the cycle of existence. Outside ourselves, no buddhas, according to Master Hakuin.  

Master Dogen’s masterful exposition of the relationship of being to time in Uji (The Time Being) in Shobogenzo may be extended somewhat by inserting the modern usage of spacetime from Einstein’s theory of relativity, which Uji is said to have anticipated. If we see that being is time, and time is being, we can understand that the conceptual idea of time outside of being has no validity. If there is no being, and no beings to experience time, then time can have no existence, or is devoid of meaning. If we say instead that being is spacetime, we can see also that what we call space and time are likewise not separable, and what we call being is meaningless outside of spacetime.

This is far from an exercise in massaging words, when we come down to the single point in spacetime that is our being. My central nervous system occupies a unique dharma location in spacetime, as does yours. All beings, insentient as well as sentient, occupy dharma positions in the spacetime web  absolutely unique to each. No one else, no other being, can occupy the dharma location that another being occupies.
When we sit in zazen, becoming increasingly still, this becomes increasingly apparent. It is nothing new, but something we may have forgotten. It seems to belabor the obvious to bring it up, or to even notice it. But this is the simplicity of the teachings of Buddhism. We cannot get any simpler, as an instruction, than that of observing the breath. We cannot reduce the simplicity of zazen any further, than the simplicity of sitting upright.

To return to our normal, non-interacting, state of being, we must take refuge from our own precious ideas and opinions about being. To recover our birthright, we must become as if stillborn. In order to uncover our original nature, we must allow our usual, reactive nature to subside. To accede to Buddha’s wisdom, we must abandon conventional wisdom.

To exist in a steady state of being, we must stop becoming. To stop becoming, we must relinquish all attachment to intention. Since we cannot do that intentionally, we must instead just sit. If we just sit — still enough, long enough — we may finally see for ourselves, along with Shakyamuni, that now, indeed, there is no more becoming. What remains is, must be, pure (non-dual) being.