The Zen of it.

They all agree, it seems.

This could apply equally as well to training as to life.

Do your work and step back,
The only path to serenity.
Laozi
570-490 BCE, Chinese Philosopher, Founder of Daoism
Tao Te Ching: A New English Version, Stephen Mitchell, tr., 1988

The Master sees things as they are, without trying to control them.
He lets them go their own way, and resides at the center of the circle.
Laozi
Daode Jing

Whatever feelings arise – whether pleasant, unpleasant, or neutral – abide contemplating impermanence in those feelings, contemplate fading away, relinquishment, letting go of all those feelings. Contemplating this one does not cling to anything in this world. When not clinging, there is no agitation. When not agitated one personally attains Nibbana.
Buddha
c. 563-483 BCE, Indian Prince, Mystic, Founder of Buddhism

One who has finally learned that it is in the nature of objects to come and go without ceasing, rests in detachment and is no longer subject to suffering.
Ashtavakra Gita
Ancient Sanskrit Sacred Text

You are only to perform your duty without an eye on their fruits.
Bhagavad Gita, II.70
400 BCE, Sanskrit Poem in Mahabharata, Sacred Hindu Text

Non-attachment is self-mastery: it is freedom from desire for what is seen or heard.
Patanjali
c. 200-150 BCE, Indian Yogi
How to Know God: The Yoga Aphorisms of Patanjali, 15
Swami Prabhavananda and Christopher Isherwood, trs., 1953, 1981

How to live the good life? The ability is in your soul, as long as it remains unattached to things that are morally neutral to it. And the soul will remain unattached if it carefully scrutinizes each of these neutral things both as a whole, and by separation into the elements that compose them. Remember that none of these things are responsible for creating our conception about them; these things are motionless and so can’t even approach us. It is we ourselves who create ideas about things, and, as we might say, drag them inside ourselves. It is in our power not to include them, and even if these conceptions have unconsciously gained admission to our minds, to erase them.
Marcus Aurelius
121-180, Roman Emperor, Stoic Philosopher
The Spiritual Teachings of Marcus Aurelius, Mark Forstater, tr., 2000

The six supernormal faculties of the enlightened are the ability to enter the realm of form without being confused by form, to enter the realm of sound without being confused by sound, to enter the realm of scent without being confused by scent, to enter the realm of flavor without being confused by flavor, to enter the realm of feeling without being confused by feeling, to enter the realm of phenomena without being confused by phenomena.
Linji Yixuan
d. 867, Chinese Chan Master, Linji (Rinzai) School Founder
in Zen Essence: The Science of Freedom, Thomas Cleary, tr. & ed., 1989

Once you realize universal emptiness, all situations are naturally mastered. You have perfect communion with what is beyond the world, while embracing what is within all realms of being.
Fenyang Shanzhao
947-1024, Chinese Chan Buddhist Master
in Zen Essence: The Science of Freedom, Thomas Cleary, tr. & ed., 1989

Just detach from thoughts and cut off sentiments and transcend the ordinary conventions. Use your own inherent power and take up its great capacity and great wisdom right where you are.
Yuanwu Kekin
1063-1135, Chinese Chan Master
Zen Letters: Teachings of Yuanwu, J. C. Cleary & Thomas Cleary, trs., 1994

Live in the nowhere that you come from,
Even though you have an address here.
Jalaluddin Rumi
1207-1273, Afghani-Turkish Sufi Mystic, Poet

Many times the mountains
Have turned from green to yellow.
So much for the capricious earth!
Dust in your eyes,
The triple world is narrow;
Nothing on the mind,
Your chair is wide enough.
Muso Kokushi
1275-1351, Japanese Zen Master, Calligrapher, Poet

Desire nothing, and you’re content with everything
Pursue things, and you’re thwarted at every turn.
Ryokan
1758-1831, Japanese Zen Master, Poet, Calligrapher

Detachment is not indifference. It is the prerequisite for effective involvement. Often what we think is best for others is distorted by our attachment to our opinions: we want others to be happy in the way we think they should be happy. It is only when we want nothing for ourselves that we are able to see clearly into others’ needs and understand how to serve them.
Mahatma Gandhi
1869-1948, Indian Spiritual Leader

By detachment I mean that you must not worry whether the desired result follows from your action or not, so long as your motive is pure, your means correct.
Gandhi
in Eknath Easwaran, Gandhi, The Man, 1997

To renounce things is not to give them up. It is to acknowledge that all things go away.
Shunryu Suzuki
1905-1971, Japanese Zen Master

To desire in the void, to desire without any wishes. To detach our desire from all good things and to wait. Experience proves that this waiting is satisfied. It is then that we touch the absolute good.
Simone Weil
1909-1943, French Philosopher, Essayist, Mystic
in The Enlightened Mind: An Anthology of Sacred Prose, Stephen Mitchell, ed., 1991

My soul does not find itself unless it acts. Therefore it must act. Stagnation and inactivity bring spiritual death. But my soul must not project itself entirely into the outward effects of its activity. I do not need to see myself, I merely need to be myself. I must think and act like a living being, but I must not plunge my whole self into what I think and do, or seek always to find myself in the work I have done.
Thomas Merton
1915-1968, American Trappist Monk, Writer
No Man Is an Island, 1955

From the moment that a man no longer responds in the slightest to the motives that regulate the material world, that world appears to be at complete repose.
Yukio Mishima
1935-1970, Japanese Writer
“The Priest of Shiga Temple and His Love,”
Death in Midsummer and Other Stories, 1966

To become free of attachment means to break the link identifying you with your desires. The desires continue: They are part of the dance of nature. But a renunciate no longer thinks that he is his desires.
Ram Dass
1931-, American Psychologist, Teacher, Writer
Be Here Now, 1971

Looking at life’s situations from a distance is the first step toward finding a solution and preventing them from happening again. By separating yourself from your experiences, you are able to move on with your life. If you don’t, you’re stuck in the puddle of the past without a paddle.
Famous Amos
1937-, African-American Athlete, Entrepreneur
Watermelon Magic, 1996

Much of our inner turbulence reflects the fear of loss: our dependence on people, circumstances, and things not really under our control. On some level we know that death, indifference, rejection, repossession, or high tide may leave us bereft in the morning. Still, we clutch desperately at things we cannot finally hold. Nonattachment is the most realistic of attitudes. It is freedom from wishful thinking, from always wanting things to be otherwise.
Marilyn Ferguson
1938-, American Writer, Mind Researcher
The Aquarian Conspiracy, 1980