As evidenced by echoes.
That wheels do not press down the ground.
Touching only at a point.
That the eye does not see.
It is the man.
That the finger does not touch. That the uttermost extreme is not the end. That a tortoise is longer than a snake.
Because longer lived!
That a carpenter's square is not square.
Like Horace's Whetstone which makes other things sharp, "exsors ipsa secandi."