Mind does not know mind;
having a mind, one does not see mind.
Producing thought is confusion;
no thoughts is nirvana.
The Buddhas attain liberation by way of mind;
the mind, without defilement, is called pure:
immaculate in all states of being, it does not take on form.
Those who understand this attain the great Way.
Reference: Stopping and Seeing: Comprehensive Course in Buddhist Meditation