There are two beautifully descriptive phrases which appear again and again in Zen literature: Mizu no kokoro and Tsuki no kokoro.
Mizu no kokoro is often translated as “mind like water.” This is a lovely phrase which is too inexact to be very helpful, standing alone. What is meant by this is to make the mind calm when facing an emergency or an adversary. The calm mind, like still water, accurately reflects all that comes before it. It is otherwise referred to as fudoshin or “immovable mind.”
Tsuki no kokoro is usually translated as a mind like the moon. This refers to the necessity of maintaining surveillance over one’s surroundings at all times. As the bright illumination produced by the unclouded full moon as it reflects its light earthward, so the mind must be aware of all conditions surrounding it. This is often described as zanshin or kan-ken futatsu no koto, or “perceiving with both the eyes and the intuitive mind.”