'As a bird when tied by a string flies first in every direction, and finding no rest anywhere, settles down at last on the very place where it is fastened, exactly in the same manner, my son, that mind (the gîva, or living Self in the mind, see VI, 3, 2), after flying in every direction, and finding no rest anywhere, settles down on breath; for indeed, my son, mind is fastened to breath.

In plain words: A bird tied by a string flies everywhere, finds no rest, and finally settles at the spot where it is tied. In the same way, the mind runs in every direction, finds no rest, and finally settles on the breath, because the mind is tied to the breath.