What is the Tao?

“The Tao that can be understood  is not the eternal, cosmic Tao,  just as an idea that can be expressed in words is not the infinite idea. And yet this ineffable Tao  is the source of all spirit and matter; expressing itself,  it is the mother of all created things. Not to desire material things  is to know the freedom of spirituality; and to desire them is to suffer  the limitations of matter. Yet these two things, matter and spirit, so different in nature, have the same origin. This unity is the mystery of mysteries, and the gateway to spirituality.” Chapter 1