Was Dōgen's teaching missing was the last Zen slogan?

In an older blog I wrote that Zen’s history tells us that Dōgen Zenji, around 1227, a former Tendai student, had studied and received transmission in China in the Caodong tradition under Zen master Tiantong Rujing who was a third-generation disciple of Zhenxie Qingliao.  All this came under the heading of a meditation called ‘silent illumination’ which was then the main practice of Caodong. 

Satisfied that he had reached enlightenment Dōgen left China in 1227 or 1228 and returned to Japan to establish the Sōtō sect曹洞宗.  But what he brought back with him from China was not a Zen based on the four slogans of Zen attributed to Bodhidharma.  They were:  

A special transmission outside the teachings,
do not depend on written words,
directly point to the human mind,
see one’s nature and become Buddha.

What Dōgen was missing was the last slogan, the critical part being “see one’s nature” 見性, which in Japanese is kenshō.

Surprisingly, the four Zen slogans don’t mention anything like silent illumination or zazen which is a physical exercise.  Authentic Zen practice takes place in the human mind not in the body that came through the birth canal. 

We learn about the meditation Dōgen learned in China from the teachings of Qingliao, of whom Dōgen’s teacher Rujing was a third-generation disciple.  He taught,

“Without taking a step you should constantly sit in your rooms and just forget about the teachings.  Be like withered wood, or a stone, or a wall, or a piece of tile, or a pebble. Cut off knowing and understanding and be naturally vacuous and completely bright. You should not make the least bit of conscious effort here” (M. Schlütter , How Zen Became Zen, 154).

Nothing is said about the mind.  Qingliao seems to be suggesting that we make our body into a lifeless thing.  But how is that anything like Siddhartha’s awakening whereby he became a Buddha?

The reason that the third Zen slogan says “directly point to the human mind” is because this is Zen’s terrain where the journey takes place.  The goal of the journey is with the sudden and complete cessation of the conditioned human mind or the same, mental activity.  This is gnosis of our true nature or essence which is unconditioned.  It's like knowing all waves are compositions of water.  This is also the state of being a Buddha or buddhatā.