"What is the criterion of the rightly transmitted Buddha-dharma?"
Dogen stated [in the BENDOWA]:
The Buddha and Tathagatas have an excellent way unequalled and natural to transmit the wondrous Dharma through personal encounter and to realize supreme enlightenment. As it is imparted impeccably from Buddha to Buddha, its criterion is the samadhi (*) of self-fulfilling activity (jijuyu-sammai).
For playing joyfully in such a samadhi (kono zammai ni yuki suruni), the upright sitting position in meditation is the right gate.
[...] In the context of Dogen's thought, the Samadhi of self-fulfilling activity signified the Samadhi that at once negates and subsumes self and other.... It referred to a total freedom of self-realization without any dualism of antitheses. The critically important point to note is that in Dogen, opposites or dualities were not obliterated or even blurred; they were not so much transcended as they were realized. The total freedom in question here was that freedom which realized itself in duality, not apart from it.
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(*) samadhi, sammai (三昧): "Concentration. Serene, settled, collected state of mind." (Moon in a Dewdroop). "It has been described as a non-dualistic
state of consciousness in which the consciousness of the experiencing subject
becomes one with the experienced object." (Wikipedia)