There are two witnesses, not one.
One is a function of mind when we deliberately begin paying attention to anything, such as emotions, thought, the breath, etc. This is individualistic, the individual is deliberately making an effort to attend.
Then there is the “second” witness, this is what Nisargadatta calls the principle that can be aware of knowing, or being consciousness of anything, and also of not knowing. This is the universal ability to know found in all sentient beings, and is “prior to” ontologically, the individual witness.
What we try to do in spirituality is allow the deliberate witnessing we do with the mind to finally give away to the effortless witnessing of internal processes and one’s individual reactions to the external world without any impediment whatsoever. The deliberate witnessing exposes that about which we were not conscious before, expanding our awareness to include deeper and broader parts of ourselves, which results in a relaxing of our minds, into a deep acceptance of whatever arises.