If we hold our hand close to our eyes, we would not be able to see the lines on our palm – all we would see is darkness. Only when we remove it and look at/witness it from a distance will we be able to see the distinct patterns.
A witness in life is someone who is able to observe what is happening from a distance – emotionally, physically, spiritually. Witnessing means being able to see the details which someone too deeply entangled might miss.
“当局者迷，旁观者清” – whoever came up with this ancient idiom definitely knew the importance of cultivating the attitude of a witness.
In clinical work, this would be similar to the third way of playing/listening – as a witness of what your body is doing and creating.
And in so many other aspects of life. Just be still, and witness.