Mizu No Ko-Koro

“Mizu No Ko-Koro” (水の心, in Kanji characters) is a Japanese phrase that translates into “Mind like water”. It refers to the state of displaying calmness in the face of adversity, not over/under-reacting to disturbances (i.e. the ripples in the water is merely representative of the disturbance), and return back to the calm once the disturbance is over.

Louis Frederic (Frederic & Crompton, 1998) translated and defined it as:

“A mind (spirit) like water. The perfect calm which the mind or spirit can find, producing a non-aggressive state and a feeling of "passive" resistance. Whoever possesses Mizu-no-Kokoro is thus sensitive to all impressions, just as water is sensitive to the slightest breath of wind, and his or her Ki is in harmony with all beings.”

Bibliography
Frederic, Louis & Crompton, Paul (1998). A Dictionary of Martial Arts. Tuttle Publishing, North Clarendon, VT, USA.