For the correct way of sitting upright we usually think of the detailed instructions on the physical posture and mental attitude found in Zazen Manuals (Zazengi). Dogen calls the condition of the mind and body realized in sitting upright "body and mind dropped off." It is generally referred to in Buddhism as Nirvana or Sunyata. This ultimate way of sitting upright means entering Nirvana without destroying the body given to you by your parents. As the ultimate way of sitting upright, Nirvana is not something that can only be attained after the death of the body. On the contrary, it is Awakening in this world.
The sitting upright, in which we Awaken in this world, must not be limited to the physical posture and composed mind prescribed in Zazen Manuals. As the expression "Dharma is free from form" indicates, there is no physical form, no ideas, no mind or consciousness here. In short, it has to be formless in terms of both body and mind.
Thus, in the "Disciples" chapter Vimalakirti scolds Sariputra, who is sitting quietly in the woods, saying "Sitting in complete repose means body and mind not appearing in the three worlds." Body and mind not appearing in the three worlds means that, being formless, there is no mental or physical form whatsoever. This is what I often call Nothingness Awakened to in the East (toyoteki mu), Nothingness that is Self, or Self that is Nothingness.
Since there is no body or mind, neither is there inside or outside, nor anything in between. Vimalakirti describes it as "Not abiding in the mind, nor outside of it; this is called sitting in complete repose." Truly sitting upright is free of inside and outside, so there remains nothing limited or determined. It is nothing whatsoever, not a thing at all. That is, the sitting-upright Self is beyond all determination; thus it can be absolutely free and self-determining. Being nothing at all, it is free from everything -- from life and death, evil passions, from all conditions, even from the various Buddhas. This is what Buddhism calls the Emancipated Self. Sitting upright enables one to become liberated from all bonds. This is human liberation in the ultimate sense. Through this sitting upright we can become Awakened persons who actually realize the Buddha nature.
Various Buddhist expressions corresponding to the via negativa, such as "Returning to the Origin" in Kegon Buddhism, "The Going Aspect" in Pure Land Buddhism, and "The Gate of Sweeping Away" in Zen, are no other than sitting upright in this sense. The simple expression opening our society's Vow of Humankind: "Calm and composed/ Let us awaken to our True Self," refers to this very Self-Awakening realized in sitting upright.
However, if the Self in sitting upright has merely dropped off body and mind, or simply does not let body and mind appear in the three worlds, it cannot yet be called true zazen. This is because the above-mentioned Self that is Nothingness, dropping off body and mind and sitting in complete repose, must actually be working as the Self. In Buddhism, Nirvana and Sunyata are sometimes considered simply as quietness or serenity without mental or physical substrate. But that is merely a vain sunyata, a barren nothingness, far from true Nirvana. If Nirvana is only that, it would have nothing to do with actual human activities and would be just an escape.
Sitting upright is often misunderstood in this way too, and Zen condemns it as the evil zen of silent illumination, or as dwelling in the devil's cave. Soto Zen has a tendency to degrade into such a practice. That is why the Chinese Zen master Tahui (Jp. Daie Soko, 1089 - 1163) denounced it as the evil zen of silent illumination, and more recently the Japanese Zen master Hakuin severely condemn it. Such zen is indeed the zen of the dead. Sitting so quietly without moving hands or legs, such corpses really are no different from trees or stones.
Zen records sometimes describe the condition of zazen as being "like trees and stones," but this is mean to express the no-self aspect of body and mind dropped off; it does not give the whole picture of the sitting-upright Self. This no-self "like trees and stones" must actually be one's Self that is Nothingness, becoming all kinds of ideas to work in real situations. This can then be the Self as source of all actual activity. This Self which has dropped off body and mind must be the utterly formless subject acting in unrestricted and complete freedom.
Buddhism considers Nirvana as the Wisdom-body, which means the Self that is Awakened to Nirvana. It is called a "body" because it is Self as the source, with a potential for engaging in all kinds of activities. Vimalakirti speaks of "Realizing all activities without abandoning Nirvana." He means that Nirvana is the Self and all activity arise out of it.
Thus, sitting in complete repose is not merely a total annihilation that makes body and mind disappear from the three worlds. The essence of sitting in complete repose is that Nirvana without body and mind is realized our True Self, and from this we creatively engage in all activities in unrestricted and complete freedom. The following statement in the "Seeing Sentient Beings" chapter refers to the same thing: "All dharmas are established by being free from abiding ground." This is the true meaning of sitting in complete repose and it is in this sense that we understand "Calm and composed/ Let us awaken to our True Self."
Thus, to Awaken to the True Self is to Awaken to the Formless Self which works while creating all forms, just like the great ocean, formless in the sense that it is free form the form of the waves, makes all form of waves appear on the surface without leaving a trace. This is true sitting in which all our activities are inseparable from sitting; this is what makes it possible for us to "be the master of all situations," in the words of the Chinese Zen master Lin-chi (Jp. Rinzai Gigen, d.866).
The Song of Awakening says, "Walking is Zen, sitting is Zen, Speaking, silent, moving, staying, I am always at peace." Again Lin-chi speaks of "The Dharma named Mind penetrates all ten directions." This Zen, this dharma named Mind, are none other than the true sitting described above. Sitting which only takes place when you are actually in the seated posture and is lost when you rise is no more than a specific form of body and mind; this cannot be sitting as the source of all activity. If you really sit well, it is not lost when you engage in other activities; it deepens and becomes more secure in the midst of turbulent activity. Such a practice enables us to work in an increasingly composed manner so that we can make quick and appropriate responses to the ever-changing actualities of this world and "construct a world in which all can live truly and fully," [according to the conclusion of the FAS Society's Vow of Humankind.]
However, if sitting is, as is often thought, just a shelter to avoid the troubles of this world, or static doldrums incapable of being a creative source in world construction, then it will be an escape -- something qualitatively different from what we call sitting. Sitting must be bound by neither by anything already existing or created, nor even by its own activity of creation. Because of this, true sitting must instead be the positive, active subject-source which is forever creating history in which all can live truly and fully. Progress in science, technology, and social organization is rightly celebrated as a sign of human evolution and historical development. The frequent condemnation of this progress as the cause of dehumanization, accompanied by impotent calls to "return to nature," is due to a lack of true sitting. True sitting can no longer be confined to quiet places like mountains and monasteries, but it must be right in the midst of constructing history and the world. Constructing the world with this sitting as Self is the true meaning of sitting in complete repose: in the words of Vimalakirti, "Realizing the affairs of an ordinary person without abandoning the Dharma-Way."