Not thinking about anything is zen. Once you know this, walking, standing, sitting, or lying down, everything you do is zen. To know that the mind is empty is to see the buddha.... Using the mind to reality is delusion. Not using the mind to look for reality is awareness. Freeing oneself from words is liberation.
Whoever realizes that the six senses aren't real, that the five aggregates are fictions, that no such things can be located anywhere in the body, understands the language of Buddhas.
All Buddhas preach emptiness. Why? Because they wish to crush the concrete ideas of the students. If a student even clings to an idea of emptiness, he betrays all Buddhas.