This might sound weird, but have you ever checked to see just how sensitive the rims of your nostrils are? Well, I didn’t expect you were going to admit it!
Try touching the inner rims of your nostrils as gently as you can (check no-one is watching first!). Use the very tip of your finger, and try to find the lightest touch that you can still feel. You should find that you’re able to feel your fingertip almost before it makes physical contact. The rims of your nostrils are covered with tiny little hairs, just a fraction of a millimeter long. Each hair has a very sensitive nerve at the root, and every time your breath passes through your nostrils, these nerves are triggered.
Of course we don’t usually notice those sensations, but it’s an excellent exercise to try to be aware of the breath passing over your nostrils. Having to pay attention to such a refined sensation encourages your mind to move onto a more subtle level of perception.
And since it’s not possible to remain aware of such a subtle sensation unless your mind is very still, the fourth stage encourages deeper levels of mental and emotional stillness.