Kōans are accordingly grouped into five categories in a most fully developed system: the first group is designed for reaching li (suchness) (richi) or the body of truth (hosshin), i.e., an enlightenment experience; the second group for a linguistic articulation (gensen) of meditational experiences in order to master the skillful use of language; the third group for those kōans truly difficult to pass (nantō); the fourth group for the practitioner to make an insight of kōan experiences pertinent to daily life (kikan) in order to embody a middle-way in which the practitioner won’t be steeped either in the state of meditation or the activity of daily life; and the fifth group for going beyond the state of buddhahood by erasing all traces of enlightenment in order to achieve a traceless enlightenment (kōjō).
The Rinzai school summarizes this process of self-cultivation in four mottoes: “being a special transmission outside of the scriptures,” “having no dependence on words and letters,” “pointing directly into [one’s] human mind,” and “seeing into [one’s] nature to become a buddha.” (See, for examples, The Gateless Gate and The Blue Cliff Record.) While the first two phrases point to the fact of discovering an extra-linguistic reality that naturally opens up in meditational experience and of articulating it linguistically in the “best” way according to the capacity of an individual practitioner, the last two phrases indicate a concretization of the original or inherent enlightenment (hongaku) in the Zen practitioner, where the original enlightenment means that human beings are innately endowed with a possibility of becoming a Buddha.
On the other hand, the Sōtō school, of which Dōgen (1200–54) is the founder, does not rely on an elaborate kōan system to learn to become a Zen person, but instead follows a method called “just sitting” (shikan taza). It refers to a single-minded, diligent practice where the qualifying term “just” means the practice of meditation without any intervention of ego-logical interest, concern, or desire, so that the practice remains undefiled. This is a method of meditation predicated on the belief that the Zen practitioner engages in the practice in the midst of the original enlightenment. Or to characterize it by using Dōgen’s phrase, it is a method of “practice-realization.” By hyphenating practice and realization, the following implications are suggested: meditation is not a means to an end, i.e., a means to realization, and thereby Dōgen closes a dualistic gap, for example, between potentiality and actuality, between before and after. Accordingly, he collapses the distinction between “acquired enlightenment” (shikaku) and “original enlightenment,” (hongaku) where the “acquired” enlightenment means an enlightenment that is realized through the practice of meditation as a means. With this collapsing, the Sōtō School holds that practice and realization are non-dual in relation to each other; they are “not two.”
According to the Sōtō school, the meditational practice, when it is seen as a process of discovery, is a deepening process of becoming aware of the original enlightenment with an expansion of its corresponding experiential correlates and horizons, and it is for this reason called the school of “gradual enlightenment” or “silent illumination.” On the other hand, the Rinzai school is called the school of “sudden enlightenment,” because it does not recognize a process leading to enlightenment (satori) as something worthy of a special attention; what counts is an experience of satori only. Even though there is the above difference in approach between Rinzai and Sōtō schools, the outcome is the same for both insofar as the embodiment of wisdom and compassion is concerned. This is because they both follow the same practice of sitting meditation.
According to D. T. Suzuki,
Satori is the raison d'être of Zen, without which Zen is not Zen. Therefore every contrivance, disciplinary and doctrinal, is directed towards satori.
This view is typical of Rainzai, which emphasizes satori. The Sōtō school rejects this emphasis, and instead emphasizes "silent illumination" through the practice of zazen.