The opening Koan from the venerated Mumonkon Collection is: ‘Joshu’s Mu!’. It is in response to the question: ‘Does a dog have Buddha-Nature?’.
Joshu’s ‘Mu!’ is a negative particle, a vociferous assertion of negation. The driving kinetic of its terse formulation is its simultaneous self-consumption. Literally, ‘Nothing’.
A fiery, full-blast Self-Negating Expression.
Again from the Mumonkan:
Daibai asked Baso: ‘What is the Buddha?’
Baso answered: ‘The mind is the Buddha.’
A monk asked Baso: ‘What is the Buddha?’
Baso replied: ‘Not mind, not Buddha.’
Bodhidharman, its First [Chinese] Patriarch defined the ends of C’han-Zen Practice as follows:
‘A special transmission outside the scriptures; no dependence on words and letters; seeing into one’s Self-Nature and the attaining of Buddha-hood.’
Do note that ‘no dependence on words and letter ‘is itself an expression in word and letter.