As we proceed deeper into the subject of Mind, thoughts and consciousness, the Mind gets absorbed in actual contemplation! And the whole being travels deeper and deeper into the realms of thought! We forget the Mind, the Body and the external environment!
The experience is very similar to the one when we watch an interesting movie! We forget ourselves!
But for evolving on the mental plane, we need to be conscious of the effect of such a study on our Mind. In other words, our awareness must not be switched off like it happens when watching a movie!
This action of constantly being aware is the state of being a witness! This state describes the scriptures statement- when thine eye is single, your body will be full of light! The light is the true knowledge. Knowledge which is beyond joy and suffering. And one who is beyond joy and suffering is released from all illusion! And one who is free of all illusion, rests in his TRUE SELF!!