Some Mahasatipatthana Sutta

Buddha said,  Mendicants, the four kinds of mindfulness meditation are the path to convergence. They are in order to purify sentient beings, to get past sorrow and crying, to make an end of pain and sadness, to complete the procedure and to realize extinguishment. What for? It's when a mendicant meditates by observing an aspect of the body, keen, aware, and mindful, rid of desire and aversion for the world. They meditate observing an aspect of feelings, keen, aware, and mindful, rid of desire and aversion for the world. They meditate observing an aspect of the mind, keen, aware, and mindful, rid of desire and aversion for the world. They meditate observing an aspect of principles, keen, aware, and mindful, rid of desire and aversion for the world.  Observing the body. Mindfulness of breathing. And how does a mendicant meditate observing an aspect of the body? It's when a mendicant, gone to a wilderness or to the root of a tree or to an empty hut, sits down cross legged with their body straight and focuses their mindfulness right there. Just mindful, they breathe in. Mindful, they breathe out. When breathing in heavily, they know, I'm breathing in heavily. When breathing out heavily, they know, I'm breathing out heavily. When breathing in lightly, they know, "I'm breathing in lightly." When breathing out lightly, they know, "I'm breathing out lightly." They practice breathing in, experiencing the whole body. They practice breathing out, experiencing the whole body. They practice breathing in, stilling the body's motion. They practice breathing out, stilling the body's motion. It's like an expert carpenter or carpenter's apprentice. When making a deep cut, they know, "I'm making a deep cut." And when making a shallow cut, they know, "I'm making a shallow cut." And so they meditate observing an aspect of the body internally, externally, and both internally and externally. They meditate observing the body as liable to originate, as liable to vanish, and as liable to both originate and vanish. For mindfulness is established that the body exists to the extent necessary for knowledge and mindfulness. They meditate independent, not grasping at anything in the world. That's how a mendicant meditates by observing an aspect of the body. The postures. Furthermore, when a mendicant is walking, they know, "I am walking." When standing, they know, "I am standing." When sitting, they know, "I am sitting." And when lying down, they know, "I am lying down." Whatever posture their body is in, they know it. And so they meditate observing an aspect of the body internally, externally, and both internally and externally. They meditate observing the body as liable to originate, as liable to vanish, and as liable to both originate and vanish. They meditate independent, not grasping at anything in the world. That, too, is how a mendicant meditates by observing an aspect of the body. Situational awareness. Furthermore, a mendicant acts with situational awareness when going out and coming back, when looking ahead and aside, when bending and extending the limbs, when bearing the outer robe, bowl, and robes, when eating, drinking, chewing, and tasting, when urinating and defecating, when walking, standing, sitting, sleeping, waking, speaking, and keeping silent. And so they meditate observing an aspect of the body internally. That, too, is how a mendicant meditates by observing an aspect of the body. Focusing on the repulsive. Furthermore, a mendicant examines their own body up from the soles of the feet and down from the tips of the hairs, wrapped in skin and full of many kinds of filth. In this body, there is head hair, body hair, nails, teeth, skin, flesh, sinews, bones, bone marrow, kidneys, heart, liver, diaphragm, spleen, lungs, intestines, mesentery, undigested food, feces. Bile, phlegm, pus, blood, sweat, fat, tears, grease, saliva, snot, synovial fluid, urine. It's as if there were a bag with openings at both ends, filled with various kinds of grains, such as fine rice, wheat, mung beans, peas, sesame, and ordinary rice. And someone with good eyesight were to open it and examine the contents. These grains are fine rice. These are wheat. These are mung beans. These are peas. These are sesame. And these are ordinary rice. And so they meditate, observing an aspect of the body internally. That too is how a mendicant meditates by observing an aspect of the body. Focusing on the elements. Furthermore, a mendicant examines their own body, whatever its placement or posture, according to the elements. In this body, there is the earth element, the water element, the fire element, and the air element. It's as if an expert butcher or butcher's apprentice were to kill a cow and sit down at the crossroads with the meat cut into portions. And so they meditate, observing an aspect of the body internally. That too is how a mendicant meditates by observing an aspect of the body.