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The Kimsuka Sutta – A Swift Pair of Messengers Shamatha-Vipassana in the Pali Canon
In the Kimsuka Sutta, the Buddha uses the metaphor of a Swift Pair of Messengers to emphasize the importance of Right Meditation. For the Forty-five years of his teaching career, the Buddha taught only one meditation technique with one purpose. He taught Shamatha-Vipassana meditation to quiet the mind and develop concentration. From a well-concentrated mind, refined mindfulness can hold in mind the framework of the entire Eightfold Path. From refined mindfulness, useful insight of the Three Marks of Existence can develop. 
Keep in mind that when the Buddha taught the Dhamma, there were no divergent schools of Buddhism, only his direct teachings. The two combined aspects of quieting the mind (Shamatha) and insight into The Three Marks of Existence (Vipassana) are taught as the meditation method used as the eighth factor of the Eightfold Path known as “Right” or “Skillful” Meditation.
As can be seen in the following story, the Buddha taught that shamatha and vipassana, tranquility and insight, are both necessary components of an effective meditation practice, with an emphasis given to shamatha, quieting the mind, for insight to be meaningful. Shamatha is often spoken of as developing ”samadhi” or a well-concentrated mind state. Shamatha is also referred to in terms of developing “jhana” or a state of deep absorption. (four “levels” of jhana are often mentioned and these are simply deeper levels of concentration, or non-distraction). 
The Buddha consistently taught shamatha-vipassana with the emphasis on shamatha. This should not be misunderstood to infer that shamatha is more important or singularly more effective as a separate method of meditation, only that a quiet, tranquil mind is necessary for true insight to arise. Without a quiet, tranquil mind, discriminating or discursive views will continue to arise leading to more confusion and delusion. Without insight there can be no understanding of the causes of discriminating and discursive views, and no clear and direct path towards understanding and release from confused and deluded views.
It cannot be found that the Buddha taught only shamatha or that shamatha can be discarded at some point with only the intention of gaining insight without first quieting the mind and developing the entire Eightfold Path. Once the mind is well concentrated by awareness of the breath in the body, insight into the functioning of the mind is observed to recognize (gain insight into) discriminating and discursive thought and the effects of delusion within an impermanent, ever-changing environment.
Ultimately what is recognized is that all thought is impermanent and fleeting, and the objects of all thought are impermanent and fleeting. This leads to recognition and abandonment of conditioned thinking, allowing an unfettered mind to arise. (In the Satipatthana Sutta, the sutta on the Four Foundations of Mindfulness, the Buddha teaches as the first foundation of mindfulness of holding in mind the “breath in the body.” A talk on The Satipatthana Sutta is here: The Satipatthana Sutta) A quiet and well-concentrated mind allows insight to Impermanence, Not-self, and Stress.
Two monks of the Buddha’s Sangha were talking. One asked the other “To what extent is one’s vision said to be well-purified?”
“When one knows as it actually is the origination and the passing away of the six-sense base. It is to this extent that one’s vision is said to be well-purified.”
Dissatisfied with this answer he asks another monk the same question.
“When one knows as it actually is the origination and the passing away of the Five Clinging-Aggregates. It is to this extent that one’s vision is said to be well-purified.”
Still dissatisfied he asks another monk the same question.
“When one knows as it actually is the origination and the passing away of the Four Great Elements of earth, fire, wind, and water. It is to this extent that one’s vision is said to be well-purified.”
Still dissatisfied he asks another monk the same question.
“When one knows as it actually is that all conditioned things are subject to cessation. It is to this extent that one’s vision is said to be well-purified.”
Still dissatisfied he seeks the Buddha. He tells him of the answers to his questions.
The Buddha replies “Friend, it is as if a man had never seen a certain tree that changes appearance often – a difficult to describe tree – a ‘Riddle Tree.’ He would ask someone to describe a ‘Riddle Tree.’ They say that this tree is black as a burnt stump. This is how the tree looked at the time.
“Dissatisfied with this answer they ask another to describe a Riddle Tree. They tell him that this tree has no bark and its pods explode. This is how the tree looked at the time.
“Dissatisfied with this answer they ask another to describe a Riddle Tree. They tell him that this tree has thick foliage with dense shade. This is how the tree looked at the time.
“In this same way what these people of integrity were focused on when their vision became well-purified is the way that they answered.
“Suppose, friend, that there were a royal frontier fortress with strong walls & ramparts and six gates. In it would be a wise, experienced, intelligent gatekeeper to keep out those he didn’t know and to let in those he did. A swift pair of messengers, coming from the east, would say to the gatekeeper, ‘Where, my good man, is the commander of this fortress?’
He would say, ‘There he is, sirs, sitting in the central square.’ The swift pair of messengers, delivering their accurate report to the commander of the fortress, would then go back by the route by which they had come. Then a swift pair of messengers, coming from the west… the north… the south, would say to the gatekeeper, ‘Where, my good man, is the commander of this fortress?’
He would say, ‘There he is, sirs, sitting in the central square.’ The swift pair of messengers, delivering their accurate report to the commander of the fortress, would then go back by the route by which they had come.
“I have given you this simile, friend, to convey a message. The message is this: The fortress stands for this body — composed of four elements, born of mother & father, nourished with rice & barley gruel, subject to constant rubbing & abrasion, to breaking & falling apart. The six gates stand for the six internal sense media. The gatekeeper stands for mindfulness. The swift pair of messengers stands for tranquillity (shamatha) and insight (vipassana). The commander of the fortress stands for consciousness. The central square stands for the four great elements: the earth-property, the liquid-property, the fire-property, & the wind-property. The accurate report stands for unbinding (nibbana). The route by which they had come stands for the Noble Eightfold Path: Right View, Right Intention, Right Speech, Right Action, Right Livelihood, Right Effort, Right Mindfulness, and Right Meditation.” 
End of Sutta
As can be seen, the swift pair of messengers of tranquility and insight combined are very effective in gaining entry through the fortress of conditioned thinking and providing the skillful means, when combined with the moral and ethical practice of the Eightfold Path, recognizing and abandoning confused and deluded thinking that would otherwise continue stress and suffering. The Buddha offered no other instruction for gaining insight and understanding to the problem of craving, aversion, and delusion but The Noble Eightfold Path, including Right Meditation, shamatha-vipassana meditation. Engaging in the teachings of the Buddha is known as engaging in “The Great Matter” of understanding confusion, delusion, and the underlying unsatisfactory life experience.
The Buddha often taught to “find a tree root or abandoned hut” for seclusion to meditate and develop the other seven supportive factors of the Eightfold Path, before other activities, even altruistic endeavors. The reason for this is another teaching of the Buddha: “It is through understanding (understanding = wisdom) that one fully understands others suffering, and through compassion that they undertake to counteract it… It is through understanding that one crosses over and through compassion that one can bring others across…Likewise it was through compassion that one can become the world’s helper and through understanding that one can become one’s own helper.” 
The following are a few other sources from the Pali Canon where shamatha-vipassana is mentioned: (not an exhaustive list) “If a monk would wish, ‘May I be dear and pleasing to my fellows in the holy life, respected by & inspiring to them,’ then he should be one who brings the precepts to perfection, who is committed to inner tranquillity of awareness, who does not neglect jhana, who is endowed with insight, and who frequents empty dwellings.” 
Again, emphasizing shamatha, not neglecting jhana, and becoming endowed with insight, frequently meditating (in empty dwellings or places of solitude) “There is the case where a Dhamma practitioner has developed insight preceded by tranquillity. As they develop insight preceded by tranquillity, the path is born. They follow that path, develop it, pursues it. As they follow the path, developing it & pursuing it — their fetters are abandoned, their obsessions destroyed. Then there is the case where a Dhamma practitioner has developed tranquillity preceded by insight. As they develop tranquillity preceded by insight, the path is born. They follow that path, develop it, and pursue it. As they follow the path, developing it & pursuing it — their fetters are abandoned, their obsessions destroyed. Then there is the case where a Dhamma practitioner has developed tranquillity in tandem with insight. As they develop tranquillity in tandem with insight, the path is born. They follow that path, develop it, and pursue it. As they follow the path, developing it & pursuing it — their fetters are abandoned, their obsessions destroyed.
“Then there is the case where a Dhamma practitioner’s mind has its restlessness concerning the Dhamma well under control. There comes a time when their mind grows steady inwardly, settles down, and becomes unified & concentrated. In this one the path is born. They follow that path, develop it, and pursue it. As they follow the path, developing it & pursuing it — their fetters are abandoned, their obsessions destroyed. “Whoever declares the attainment of arahantship (awakening) in my presence, they all do it by means of one or another of these four paths.
“Then there is the case where a monk’s mind has its restlessness concerning the Dhamma well under control. There comes a time when his mind grows steady inwardly, settles down, and becomes unified & concentrated. In him the path is born. He follows that path, develops it, pursues it. As he follows the path, developing it & pursuing it — his fetters are abandoned, his obsessions destroyed. Whoever declares the attainment of arahantship (awakening) in my presence, they all do it by means of one or another of these four paths.”
The Buddha describes again the necessity of shamatha-vipassana as the mediation method leading to all “obsessions destroyed.” “As for the individual who has attained both internal tranquillity of awareness & insight into phenomena through heightened discernment, his duty is to make an effort in establishing (‘tuning’) those very same skillful qualities to a higher degree for the ending of the (mental) fermentations. 
Shamatha-Vipassana instructions: Shamatha-Vipassana meditation
An article on the authenticity of the Pali Canon is here: The Pali Canon – What the Buddha Taught
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My Dhamma articles and talks are based on the Buddha's teachings (suttas) as preserved in the Sutta Pitaka, the second book of the Pali Canon. I have relied primarily on Thanissaro Bhikkhu’s excellent and insightful translation of the Pali generously made freely available at his website Dhammatalks.org, as well as the works of Nyanaponika Thera, John Ireland, Maurice Walsh, Hellmuth Hecker, and Sister Khema, among others, as preserved at Access To Insight.
Also, I have found Bhikkhu Bodhi's translations from Wisdom Publications Pali Canon Anthologies to be most informative and an excellent resource.
I have made contextual edits to the suttas from these sources for further clarity, to modernize language, to minimize repetition, and maintain relevance to Dependent Origination and Four Noble Truths.
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