When you see breath energy in a constant way, it’s a lot easier to adjust the breath in a way that feels right, feels healthy, feels nourishing. You can gain a sense of fullness without feeling stuffed.

"Before you take up the three perceptions of inconstancy, stress and not-self, [the Buddha] has you develop other perceptions first — in particular the perceptions that lead the mind to concentration. While you’re here practicing concentration, focus in on your breath. What keeps you with the breath? A perception, a mental label that says “breath.” And a lot of the concentration practice is learning how to gain a perception of breath that you can hold in mind for long periods of time with a sense of ease, a sense of well-being. You test different perceptions of the breath to see which ones hold — in other words, which ones you can stick with.

This would seem to go against the perception of inconstancy, and it does. You’re actually looking for a perception you can hold onto as constant. You want to see the breath as something consistent and pleasant you can stay with. For example, if you see the in-breath and the out-breath as two radically different things, it’s going to be hard to stay with them comfortably because you have to keep switching back and forth between the two perceptions. But if you see the breath energy as something present in the body all the time, try to hold that perception in mind. You’ll come to sense the breathing process in a different way. The breath is always there, it’s just a matter of letting the in-breath meld with what’s already there. Then you check to see whether “what’s there” is getting pumped in too full or squeezed out too much.

When you see breath energy in a constant way, it’s a lot easier to adjust the breath in a way that feels right, feels healthy, feels nourishing. You can gain a sense of fullness without feeling stuffed. When you breathe out, you can begin to sense the point where you’ve breathed out too much: You’re squeezing the breath energy, depleting yourself of breath energy in ways you don’t need to.

For the time being, you simply want to hold onto that perception of the breath as constant and pleasant, something you can gain some control over. Use the three perceptions only when you find the mind being pulled away from the breath to other things. You look at the happiness, the pleasure that comes from chasing after those other things, and you learn to see it as less constant than the pleasure that comes in staying with the breath: more stressful, less under your control.

So at this stage in treating your illness, your primary focus is on constancy, on seeing the breath as always there. Ajaan Lee says that you want to see what’s constant in what’s inconstant. If you see everything as inconstant, he says, you’re missing some important aspects of the training. The example he gives is symbolic: Your lower lip has never turned into your upper lip. That’s something constant about your lower lip. Your eye has never turned into an ear. That’s something constant about your eye. In the same way, there are some aspects of the breath energy in the body that really are constant. As long as you’re alive, there will be breath energy in the body.

And you can learn to use that fact to your advantage. Just stick with that perception of breath and see how the underlying breath energy in the body goes through small fluctuations. You hold onto the idea that breath is constantly there, simply that it’s sometimes stuffed in too full, sometimes squeezed out until it’s too depleted. But you can learn to adjust the rate of your in-breath and out-breath, to see the in-ness and the out-ness as secondary and the presence of breath energy in the body as primary, the given you hold onto."

~ Thanissaro Bhikkhu "Medicine – Timely & Timeless" (Meditations5)