According to Osho, witnessing is both learning and remembering—and finally neither. At first you learn to enter the inner temple; then you keep remembering, since forgetfulness is human. As remembrance becomes effortless, like silent breathing, witnessing reveals itself as your innate nature: you have not achieved anything; it was always there. Be a witness and you are a buddha.
First you practice noticing, then you keep remembering to notice, until noticing happens by itself because it’s who you are.