What is “mind like water”? What does it mean to have a peaceful mind?

A collection of quotes and inspirational thoughts presented by Shodon Tim Morton:

There is a beautiful descriptive phrase, which appears again and again in Zen & Budo (the ‘way’ or the ‘path’ of Martial Arts) literature: Mizu no kokoro “mind like water”. For a better understanding, I have taken the liberty to compile various interpretations of this same phrase.
— Sensei Ty Aponte

“The appeal of “Mind Like Water” is the calmness and peace that you are trying to achieve. Have everything in its place, and empty your mind of the busy-ness and junk. Then you are ready for anything that comes your way. It is an ongoing quest.”
— Leo Babauta

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.”
— Joseph Caulfield

Calm water is like a mirror and gives a picture of all that is around. Conversely troubled waters only reflect the turmoil within. When a person approaches a problem with calm and composure then like the reflection on calm water he sees everything. So also when a problem causes turmoil then like the troubled water on confusion is seen. Imagine an intellect as calm, a will as relentless and indomitable and a personality as adaptable as water and you will have envisioned a MIND LIKE WATER.
— Sensei Takayuki Mikami

“This term was emphasized in the teachings of the ancient karate masters. It refers to the mental attitude required while facing an actual opponent. Mizu no kokoro (Mind like water) refers to the need to make the mind calm, like the surface of undisturbed water. To carry the symbolism further, smooth water reflects accurately image of all objects within its range, and if the mind is kept in this state, apprehension of the opponent’s movements, both psychological and physical, will be both immediate and accurate, and one’s responses, both defensive and offensive, will be appropriate and adequate. On the other hand, if the surface of the water is disturbed, the images it reflects will be distorted, or by analogy, if the mind is preoccupied with thoughts of attack or defense, it will not apprehend the opponent’s intentions, creating an opportunity for the opponent to attack.”
— Sensei H. Nishiyama

In karate there is an image that’s used to define the position of perfect readiness: “mind like water.” Just like water in a pond, your mind should be at peace most of the time. And it should only react when something is thrown into it. Imagine throwing a pebble into a still pond. How does the water respond? Water reacts exactly in proportion to the object thrown into it – with a big splash for a big rock, or a little ripple for a small pebble. And when the reaction is over, it goes back to a state of peace/calm. It doesn’t overreact or underreact. Anything that causes you to overreact or underreact can control you, and often does. Responding inappropriately to your e-mail, your staff, your projects, your unread magazines, your thoughts about what you need to do, your children, or your boss will lead to less effective results than you’d like. Most people give either more or less attention to things than they deserve, simply because they don’t operate with a “mind like water.”
— David Allen

As the moon shines equally on everything, so does the mind encompass all it perceives without discrimination? In Karate when one is freestyling, if there are thoughts of winning or losing, or what technique one should use, these thoughts are like clouds that pass in front of the moon blocking the light (of awareness).

So too with mind like water. Still water reflects only what is there. Thoughts are like dropping pebbles into that water, causing ripples in the mud. The distracted mind cannot respond accurately and immediately to what is happening. These concepts (mind like moon-mind like water) are important not only in the practice of Karate, in freestyling (sparring) or self-defense, but are also important to one’s everyday life.

The challenges of living demand accurate and intelligent responses. When the mind is clouded or disturbed by waves of anxious thinking, then one’s response is diminished and therefore one is not capable of meeting the challenge fully. But there is more to these concepts. The word “mind” referred to in Japanese Karate means “consciousness,” which is all of living. What we in the West refer to as “mind” is only a small part of this Japanese concept of mind. The typical Western notion of mind is the analytical, logical, intellect, a necessary tool for living, for creating science and technology.

But “mind” in “mind like moon-mind like water” has a far deeper and more profound meaning. This mind is the foundation of all consciousness; it is consciousness aware of itself. This mind is in stillness, in silence. When the intellectual mind finds its proper place in living, in action, then this order can pave the way for the larger mind to enter. It is this larger mind from which consciousness emanates. It is where all things arise and disappear into the void. It is the seat of being, the wellspring of life.

When there is a moment of forgetfulness, a stillpoint in the chaos of frenetic living, that mind may blossom. But all too often one is caught up in the smaller mind, in the frenzy of self-centered preservation, in trying to find psychological security. It is only by not knowing that one can come upon this mind. In not knowing there is intelligence. Not knowing does not mean ignorance, not thinking. Not knowing means that thought is not seeking security in itself. Mind Like Moon-Mind Like Water is a metaphor for the mind that is intelligent, a mind that is sane.
— Dr. Terrence Webster-Doyle