Lao Tzu wrote:

Looked at but cannot be seen - it is beneath form;
Listened to but cannot be heard - it is beneath sound;
Held but cannot be touched - it is beneath feeling;
These depthless things evade definition,
And blend into a single mystery.

In its rising there is no light,
In its falling there is no darkness,
A continuous thread beyond description,
Lining what does not exist;
Its form formless,
Its image nothing,
Its name silence;
Follow it, it has no back,
Meet it, it has no face.

Attend the present to deal with the past;
Thus you grasp the continuity of the Way,
Which is its essence.