The reasoning of the nonduality of ‘knower’ and ‘known’ in Zen is clearly presented in an early fascicle of Shobogenzo titled, Soku Shin Ze Butsu; This Mind is Buddha. ‘Buddha’ is ‘Butsu,’ ‘this mind’ is ‘soku shin’; ‘shin’ means ‘mind’ and ‘soku’ means ‘here-now’ or ‘this.’ ‘Soku Shin Ze Butsu’ is translated by Gudo Nishijima & Mike Cross as, ‘mind here and now is buddha.’ In this fascicle Dogen assures us:
What every buddha and every patriarch has maintained and relied upon, without exception, is just ‘mind here and now is buddha.’
This fundamental principle – This Mind is Buddha – is presupposed throughout the classic literature of Zen. Comprehensive, multifaceted Buddhist teachings on the nature and dynamics of ‘this mind’ constitute many of Buddhism’s most profound expressions, and some of its most often misunderstood and misrepresented expressions. Dogen and other Zen masters before and after dedicated much time and energy clarifying what it actually means to assert, ‘We are what we think’ or ‘mind here and now is Buddha.’ For instance, Dogen went to lengths to carefully explain that ‘this mind’ (that ‘is Buddha’) is all-inclusive; the ‘one totality’ – whatever is, is this mind, all that is, is Buddha.
Excerpt from, Zen Cosmology: Dogen’s Contribution to the Search for a New Worldview pp.93-94