That which is not seen is called the invisible.
That which is not heard is called the silent.
That which is not felt is called the formless.
Together, these things elude inquiry.
They are confused, and considered to be inseparable: What is seen is not bright, what is hidden is not dark. Stretched to infinity, it cannot be named.
It returns to nothingness: shape without shape, substance without substance. Illusory, unimaginable.
Encountering it you do not see its beginning. Following it you do not see its end.
Hold fast to the ancient path of the Tao in order to master the present.
(from Tao Te Ching, verse 14, translation by Robert Brookes)