As soon as one is continuously devoted to the hwadu without interruptions, the hwadu-samādhi is achieved, and if one conquers the hwadu through this samādhi, one will be enlightened. The hwadu-samādhi, according to the extent of thoroughness, can be divided into the three stages of movement and calm in one thusness, dream and awakening in one thusness, and waking and sleeping in one thusness. That is, according to how closely the hwadu has been continued with, one can divide it into stages. Here, even though we express it as stages, this definitely does not mean there are stages in enlightenment. The hwadu meditation is to be suddenly awakened then and therebecause it is the core (of that awakening).
Ganhwa Seon is not conquering and entering into the hwadu little by little. The hwadu is a totally tasteless cake made of iron. That cake is chewed and swallowed at a gulp. It totally does not recognize any stage or order. However, although the awakeing is momentary, the practice of the study necessarily needs a long period of time in progress. Of these three stages, if one enters into the stage of waking and sleeping in one thusness, one has come close to enlightenment. “One thusness” means “always like it is, consistent” or “maintaining a condition that is never interrupted.” If one is to talk of it in relation to the taking up of the hwadu, it “is the consistently continual devotion to it.” Movement and calm in one thusness means “to continually be devoted to the hwadu, whether the hwadu is moving or whether it is quiet.” Dream and awakening in one thusness means, “one consistently is devoted to the hwadu whether one is awake or dreaming.” And waking and sleeping in one thusness is “to be equally devoted to the hwadu whether one is awake or whether one is sleeping deeply.” When one investigates the hwadu, one is only awakened when one is solely devoted to the hwadu no matter whether in movement or calm, or in deep sleep, or even sleeping or awake.
Seon Master Taego Bou made this principle clear as follows:
If one can know it once in one day without interruption, and one persistently prepares the spirit even more and examines it moment by moment, then one must (do this) day by day without interruption. If for three days one can, according to the Dharma, be without interruptions or gaps, even when moving or sitting peacefully one is consistent (movement and calm in one thusness), even when speaking or is silent one is consistent, the hwadu is always present in front of one, this is like moonlight in fast-moving rapids, which even though it is dashed against (that water) is not scattered and dispersed, and even though it is bent it does not disappear. (So) if whether asleep or wake one is consistent (waking and sleep in one thusness), the time when one is greatly awakened has come close. (Taego Hwasang eorok).
Very deep sleep is a sleep that is like death. However, even at the moment of this death the mind unconsciusly moves deep within us. In Vijñānavāda this is called the ālayavijñāna. The moving mind makes people be reborn. The words, “As soon as one enters sleep, (the mind) disappears, so how can it oppose birth and death!” mean just that.
Heavy sleep is deep sleep without dream. Only when one is devoted to the hwadu, is the hwadu not interrupted even in the deep sleep and one can consistently succumb to it. Only when one is perfectly devoted to the hwadu even in deep sleep, will one definitely not retreat from the hwadu, and a time will arrive that is good and imminent. Just as Seon Master Dahui said, because the way of the waking and sleep in one thusness cannot be shown to others, it has no other (way) than to be experienced personally.