At Sāvatthī.

There the Buddha … said:

“Mendicants, do you develop mindfulness of breathing?” When he said this, Venerable Ariṭṭha said to him:

“Sir, I develop mindfulness of breathing.”

“But mendicant, how do you develop it?”

“Sir, I’ve given up desire for sensual pleasures of the past. I’m rid of desire for sensual pleasures of the future. And I have eliminated perception of repulsion regarding phenomena internally and externally. Just mindful, I will breathe in. Mindful, I will breathe out. That’s how I develop mindfulness of breathing.”

“That is mindfulness of breathing, Ariṭṭha; I don’t deny it. But as to how mindfulness of breathing is fulfilled in detail, listen and pay close attention, I will speak.”

“Yes, sir,” Ariṭṭha replied. The Buddha said this:

“And how is mindfulness of breathing fulfilled in detail? It’s when a mendicant has gone to a wilderness, or to the root of a tree, or to an empty hut. They sit down cross-legged, with their body straight, and establish 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.’ … They practice like this: ‘I’ll breathe in observing letting go.’ They practice like this: ‘I’ll breathe out observing letting go.’ This is how mindfulness of breathing is fulfilled in detail.”