The first foundation of mindfulness is mindfulness of the body including physical sensations and the breath.
The posture we take during meditation fosters alertness and serves the mind-body connection. We discover that many body sensations arise from constructs of the mind and the mind is the root of much of physical discomfort. Over time we begin to become aware of ever subtler tensions in the body and learn to open to and soften them.
We can use the body sensation of breathing as an object of concentration as it is always with us, ever-present and uninterrupted, as long as we are living. Thoughts and other distractions come and go, but we can always return to the breath. In addition, focusing on the breath has a calming effect for many people (though not for everyone) and so it is a useful starting place in learning to settle the mind.
It is written in the Mahasatipatthana Sutta (The Greater Discourse on Steadfast Mindfulness) that the Buddha instructed monks in mindfulness of the body in the following way:
… a monk, having gone to the forest, or to the foot of a tree, or to an empty, solitary place; sits down cross legged, keeping his body erect, and directs his mindfulness. Then only with keen mindfulness he breathes in and only with keen mindfulness he breathes out. Breathing in a long breath, he knows, “I breathe in a long breath; breathing out a long breath he knows, “I breathe out a long breath”; breathing in a short breath, he knows, “I breathe in a short breath; breathing out a short breath he knows, “I breathe out a short breath”, “Aware of the whole breath body, I shall breathe in”, thus he trains himself; “Aware of the whole breath body, I shall breathe out”, thus he trains himself. “Calming the process of breathing, I shall breathe in”, thus he trains himself; “Calming the process of breathing, I shall breathe out”, thus he trains himself… Thus he dwells perceiving again and again the body as just the body, not I, not mine, not self, but just a phenomenon in himself…
We watch the breath, moment by moment, non-judgmentally and with curiosity, from the start of each inhale to the finish of each exhale and the spaces in-between. Without changing the breath in any way, we notice its qualities – its physical sensations of expanding and contracting, entering and exiting – its shifting rhythms and patterns, from breath to breath, day after day. Concentration is strengthened paving the way for deeper levels of meditation. With practice we develop greater wisdom about our habits and reactions that leads us ever closer to clear seeing and liberation.