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’.
[I think it was Hakuin who wrote that all of Zen was contained in Mu]
Bodhidharman 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.’
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.’
Note that the better Koans never mark an identifiable terminus. It dates back to the founding of the Tradition itself.