In answer to Arjuna’s question about the Field and its Knower, Krishna says: ‘This body is called the Field, because a man sows seeds of action in it, and reaps their fruits. Wise men say that the Knower of the Field is he who watches what takes place within this body. Recognise me as the Knower of the Field in every body. I regard discrimination between Field and Knower as the highest kind of knowledge.’
Krishna gives us this information in Chapter 13 of the Gita. The ‘Knower of the Field’ is what we often describe in yoga as the observer, the onlooker or the silent witness.