A famous passage in the Nirvana Sutra reads:
The Buddha’s True Dharma Body is just like space
Nirvana sutra
Manifesting form according to circumstances
It is like the moon in water.
I think this is a description of our state in zazen.
“the moon in water” is a metaphor for emptiness. It isn’t that the reflection doesn’t exist, it’s that it isn’t separate. The reflection is the expression of all the different aspects of the whole of reality working together: the moon, the water, the clouds, the clouded water eyes of the person witnessing it into being.
Each line informs the other. The third line is a poetic instance of the general statement of interdependence in the second.
And the purpose of the first line is to convey that “space” and “expression” are not in conflict: our aim is not to nullify whatever arises: thoughts, noises inner and outer, feelings, but not to fixate on them either, and thus to allow vast compassionate space to manifest, a space which can hold all this expression, like a mother holds her baby.