Why yoga begins with breath
The ancient yogis observed this long before the language of neuroscience existed. They noticed that when the breath became restless, the mind often became restless too. When the breath slowed and steadied, the mind tended to follow. Rather than attempting to control thoughts directly, they learned to work with the breath first.
The mind can be difficult to influence directly. The breath is more accessible. It can be observed, regulated, lengthened, softened, and balanced. Through it, we gradually influence the body, the nervous system, and the movement of prāṇa itself.