Daoxin suggests five principles for Chan practitioners, among which the first two emphasize the advance knowledge on the essence and function of
mind.
THE FIVE GATES of DAOXIN
一者 知心体 体性淸淨 体與佛同
二者 知心用 用生法寶 起作恒寂 萬惑皆如
三者 常覺不停 覺心在前 覺法無相
四者 常觀身空寂 內外通同 入身於法界之中 未曾有碍
五者 守一不移 動靜常住 能令學者 明見佛性 早入定門
First, one should know the essence of mind. It means realizing that the fundamental of our mind is originally pure and is the same as the Buddha’s. (一者 知心体 体性淸淨 体與佛同.) Not however the same as the Chan Contemplation access.
Second, one should know the function of mind, by means of which all the Dharma treasures arise; even if they do, they are completely empty; all kinds of doubts are the same as well. (二者 知 心 用 用生法寶 起作恒寂 萬惑皆如.) Not however he same as the Chan Contemplation function.
Third, one should always be enlightened. The enlightened mind is present before but the enlightened phenomenon has no form. (三者 常覺不停 覺心在前 覺法無相.)
Fourth, one should always observe that the body is completely empty. Though the body exists in the Dharma realm, it has no hindrance, interpenetrating between inside and outside. (四 者 常觀身空寂 內外通同 入身於法界之中 未曾有碍.)
Finally, one should maintain oneness, without being disturbed. (五者 守一不移.)
The oneness mentioned above indicates a whole-heartedness with which one pays attention to the emptiness of body. Those who follow this way can see their Buddha nature and enter the gate of extinction (calmness). In this way, (動靜常住 能令學者 明見佛性 早入定門)
After teaching the five gates of his system, he continues,
One should know this－namely, that one’s self is like the moon in the water, like a reflection in a mirror, like a shimmering heat wave, or like an echo resounding in a hollow valley.
It may exist, but it can’t be investigated; it may not exist, but nevertheless it always shows itself clearly.
The Buddhas’ Dharma body is all the same as this.
So, one can know the following; one has never lived nor has been dead since infinite eons ago. If one can only gain an insight into this, that is the true repentance and the extinction of bad karmas accumulated for countless eons.
Daoxin mentions that the Buddhas’ Dharma body and the selves of the ordinary, unenlightened are not separate permanently but all alike. In other words, they can’t be defined either as being nor non-being, though evidently visible. Or the recognition of them can't be objectified or reified.