To spell out in a bit more detail what this “one taste” refers to, we’ll move now from Laozi (in his home in 4th century BCE China) ..... fast-forward about 1800 years, and drift westward several hundred miles into Tibet, where we’ll run into Longchenpa -- a 14th century CE Buddhist scholar/adept. Longchenpa’s vast and profound repertoire of written work includes a thin little volume called The Four-Themed Precious Garland -- which includes some beautiful passages describing the unchanging nature of Dharmakaya, which perhaps can shed some light on the meaning of pu kai in the context of the Daodejing.
This version of The Four-Themed Precious Garland was translated by Alexander Berzin, with commentary by Dudjom Rinpoche. I’ve posted the entire relevant excerpt here, which I strongly encourage you to read. It’s really quite amazing, for its clarity on a variety of important points. A slightly different translation of the entire text (without the commentary) can be found at Berzin Archives.
For our purpose here, of understanding what the word “unchanging” might refer to, when used as an attribute of the ultimate reality, consider the following passage, from Longchenpa’s text (emphasis added):
“The supreme peerless vehicle of the secret Dzogchen, the Great Completeness, functions to bring you directly into the Sphere of that which is spontaneously there. This sphere, which is the foundation, is unchanging. All good qualities (appear) in it spontaneously as the sun, moon, planets and stars do in the sky. It need not be sought for because it is spontaneously present from time immemorial. No trying or effort is required. This path is naturally obvious.
The mandala sphere of Clear Light is unconditioned. It is the innate Dharmakaya, the all pervasive intentionality of the Buddhas. To realize it is the supreme view of reality.”
Dudjom Rinpoche’s commentary on this passage is as follows:
“The foundational sphere or ‘founding stratum,’ which exists primordially, is identical with pure awareness. It never changes and is thus likened to the sky or space. In this space the various Buddha Bodies, qualities and pristine awareness arise spontaneously just as the sun, moon and so forth appear in the physical sky without being sought for. The planets and stars do not come about through your efforts, yet you see them. Likewise, without deliberate effort you can directly perceive the obvious path of Voidness, because Voidness by nature is directly perceivable. Dzogchen, then, is the path of Voidness itself.”
It’s worth pointing out here that the limitation of the space or sky metaphor (as a description of Dharmakaya) is that the “space” of Dharmakaya is not only infinitely spacious but also is self-aware -- hence the term “pure awareness” as a synonym for Dharmakaya.
Our conventional, dualistic (and typically Newtonian) notion of space imagines it to be a more-or-less neutral and inert stage upon which the action of the self/world unfolds (or sky within which the planets orbit). Within such a scheme, each sentient being is believed to have its own unique point of view, its own perspective, via its particular cognitive/perceptual apparatus -- with which it forms its perception/experience of the single “space” they all share.
A self-aware space, on the other hand, is not at all neutral or inert -- but rather is the sum-total of all perspectives -- an “integration” if you will (even in the mathematical sense) of all the seemingly-individual points of view.
Earlier in the same chapter of The Four-Themed Precious Garland, Dudjom Rinpoche offers -- in his commentary on Longchenpa’s presentation of the Two Truths -- a description of the unchanging-yet-far-from-inert nature of Dharmakaya as including the “one taste of the absence of all mental fabrications”:
“The appearance of all things of samsara and Nirvana is based on the causal processes of interdependent origination. These appearances are undeniable. They are the unceasing natural play of Voidness, like the play of light, the play of eddies and waves on the water and the rustling of the leaves. All things of relative reality are this way. Thus because appearances are irrefutable, there is the relative or operative level of truth.
On the ultimate level there are no validly cognized objects other than the one taste of the absence of all mental fabrications. Ultimate reality transcends the division into subject and object. It is the underlying stratum, the unborn, pure mode of existence of the appearances of the relative level. Thus the two levels of truth are inseparable.”
Can the unchanging nature of Dharmakaya/Tao -- as the “one taste of the absence of all mental fabrications” -- be experienced? Presumably it can, in some fashion, otherwise it would not be referred to -- via a sensory idiom -- as “one taste.” Nor would Dudjom Rinpoche have described it as a "validly cognized object." The very term points to a vibrant aliveness inherent in the “space” devoid of mental fabrications: an aliveness which is indeed experienced, felt, tasted. When the “background” of Dharmakaya/Tao soaks so fully into the “foreground” (of appearances) as to precipitate something like a figure-ground reversal -- there’s a distinct experience, a “taste” or "sense" or "fragrance" that accompanies this, which saturates each and every seemingly-distinct appearance arising within it.
But then this raises the question: Is this “one taste” something that is registered by the human bodymind as such -- e.g. via a nondual aspect of the nervous system? Or is the “cognizer” of the “experience” (e.g. self-cognizant jnana with suchness as its nondual "object") something fundamentally distinct from the human bodymind?
But of course .... this distinction (itself dualistic) is rendered meaningless -- simply dissolves -- within the realization of the inseparability of the Two Truths. The human bodymind perceived conventionally (i.e. dualistically) is not the experiencer of “one taste.” But the human bodymind correctly perceived, in the mode of “pure perception” -- i.e. as an expression of Dharmakaya/Tao (in the manner of waves being an expression of ocean) -- is indeed (qua Dharmakaya/Tao) the one and only perceiver/experiencer of “one taste.” The purely-perceived human bodymind (qua Body of Tao) is, in fact, an aspect of "one taste" from both "sides" if you will: simultaneously the taster and the taste, the dancer and the dance. Or, as T.S. Eliot so beautifully puts it:
At the still point of the turning world. Neither flesh nor fleshless;
Neither from nor towards; at the still point, there the dance is,
But neither arrest nor movement. And do not call it fixity,
Where past and future are gathered. Neither movement from nor towards,
Neither ascent nor decline. Except for the point, the still point,
There would be no dance, and there is only the dance.