The Zen legend begins with Bodhidharma, but it actually should begin with the Lanka School and its Lanka masters which make up the Lanka lineage beginning with Bodhidharma (or Gunabhadra) followed by Hui-k'o, Seng-ts'an, Tao-hsin, Hung-jen and Shen-hsiu rather than Hui-neng. It is from this tradition, I would argue, that the idea of the koan emerges which might very well be the brain child of Ma-tsu.
Ma-tsu came from the Lanka School which was in Szechuen. He later studied under the Sixth Patriarch's disciple Huei-jang. According to Dr. Hu Shih, "The Lanka Sutra had taught that words were not necessary to express the truth and that any gesture or motion or even silence might be used to communicate a truth. Ma-tsu developed this idea into a pedagogical method for the new Zen."
For Ma-tsu, human actions were the function (yung) of our Buddha-nature. Knocking a student to the ground, snapping a finger, coughing, smiling, pinching a student's nose, paradoxical answers, were the function of Buddha-nature. In other words, to be natural was the Way insofar as all human functions originated from Buddha-nature.
Essentially, Ma-tsu's tradition believed everything is altogether true (chen) as contrasted with Northern Zen that believed everything is altogether false (wang). What both of these traditions failed to realize, according to Zen master Tsung-mi, is that essence (t'i) is most fundamental. It is the basis upon which enlightenment is validated, rather than function. Function is only the 'branch' and not the 'root'. Realizing Buddha-nature is the root of the Bodhi-tree. It supports what is manifested of the tree above the roots.
When Joshu says “Mu” or “Go wash your bowl” this becomes a direct expression of Budhda-nature, that is, the function. But for the student who is the recipient of this oddball response to his question, it all seems strange and illogical. The student doesn’t understand that he is only getting the function, not the essence. He has to be forced to intuit the essence from the function, itself. In addition, the teacher has to disallow any kind of literary solution. The solution to each and every koan is realization of pure Mind. This is the essence which is the imprimatur—nothing else. It alone sacralizes the function. Impostors beware!