Challenging the emergent Buddha on the height of his attainment, Māra asked, "Who bears witness for you?" Siddhārtha, seated, reached out his hand, touched the ground, and the Earth itself cried out, "I bear witness!" This gesture of touching the ground is called the bhūmisparśa mudrā and is frequently used in Buddhist iconography.
I find myself stopping in my tracks whenever I consider that incident and that posture: no argument, no throng of supporters, no treatise, no choir of heavenly beings, no retort.
And his hand touching the bare earth.
And that was enough.