Well everything starts with Johari's window.
The way others perceive us and who we are in aspect that are by definition beyond us, but that nevertheless are part of who we are. Then, if that is so, grief is more than sorrow, because the nearer the person the more information they have about that non-knowable self of us. When they die, they take that parts of us with them. The very memory of us seen from a second/third person view is gone. We, intuitively know this, and the pain is almost physical.
Know thyself might be interpreted in the same way. Not only knowing yourself from your point of view but knowing yourself in the others' perception. The same for care for the other as you would care for yourself.
That is. This is the text.