Thanissaro Bhikkhu wrote:
One of the things that we want to try to do as we meditate is to get [the mind] to stay in one place, to save some energy. As long as you’re going to have a sense of self, keep it solid — rock solid — immersed in the body.
Breath meditation is one way of staying immersed in the body. The term in Pali is kayagatasati, mindfulness immersed in the body. And the quality of immersion is important. You want to fill the whole body, occupy the body, inhabit the whole body, as much as you can.
Where is your observer right now? For many of us, it’s like a weird bird perched on our shoulders and peering through our eyes. It watches the body as if the body were something separate. But as we meditate, we’re trying to get away from identifying with that particular observer; we want to be an observer filling the whole body. Your feet fill your feet, your hands fill your hands. Your entire sense of who you are fills the entire body.
This puts you in a position of strength, because if you’re leaving big gaps of unoccupied territory in your body, other things will occupy it — different thoughts, different defilements. But if your awareness occupies your whole body, other things can’t get in so easily. The image in the Canon is of a solid wooden door: a ball of string thrown at the door won’t leave a dent at all. Even if things do come in and make a dent on the mind, you’re going to know it, you’re going to see it because you’re right there. You’re not off in some other corner of the body looking at something else.
So as you focus on the breath, try to get past the idea that you’re in one part of the head watching the breath in other parts of the body. You want to occupy the whole body, bathed in the whole breath. The breath and the body should be surrounding your sense of where you are. And then you want to maintain that sense of being centered in the body like this, filling the whole body with your awareness as you breathe in, as you breathe out.
Why? For one thing, this sense of filling the body helps you stay in the present moment. When the mind goes off thinking thoughts about past and future, it has to shrink its sense of awareness, shrink its sense of itself, down to a small enough dot so that it can slip into the past or slip into the future. In other words, you latch onto the part of the body that you use as a basis for thinking about the past or the future, while other parts of the body get blotted out. But if you’re filling the body with your awareness and can maintain that full awareness, you can’t slip off into the past and future unless you want to. So this is one way of nailing yourself down to the present moment. Your inner hands are nailed to your physical hands, your feet to your feet. You can’t move.
Think of the breath coming into the *whole* body. Every cell of the body is participating in the breathing process, and you’re sitting here in the midst of it. This gives your sense of observing self a greater solidity, so that when thoughts come into the mind you’re not knocked off balance by them. You’ve got a solid foundation. The word they use for the object of meditation in Pali, arammana, literally means “support,” the idea being that your mind is standing firm on something. You’re standing here in the body. This is your location. This is where you take your stance. And when your stance is solid, nobody can kick you over or knock you down.
From: Immersed in the Body
by Thanissaro Bhikkhu