Tao Te Ching: Chapter 13
We look at it, and we do not see it, and we name it 'the Equable.' We listen to it, and we do not hear it, and we name it 'the Inaudible.' We try to grasp it, and do not get hold of it, and we name it 'the Subtle.' With these three qualities, it cannot be made the subject of description; and hence we blend them together and obtain The One.
Its upper part is not bright, and its lower part is not obscure. Ceaseless in its action, it yet cannot be named, and then it again returns and becomes nothing. This is called the Form of the Formless, and the Semblance of the Invisible; this is called the Fleeting and Indeterminable.
We meet it and do not see its Front; we follow it, and do not see its Back. When we can lay hold of the Tao of old to direct the things of the present day, and are able to know it as it was of old in the beginning, this is called (unwinding) the clue of Tao.