Vijnana Bhairava Tantra · Dharana 25 · Verse 48
Skin as an empty shell
Regard the skin as the wall of a house with nothing inside it. There is a surface, and behind the surface there is emptiness. Do not refill the interior with commentary. Stay with the empty house. If the sense of a filled-up self falls away, do not rush to rebuild it.
Older line
Think of the skin as a wall around the body, with nothing at all within it. Meditating so, you reach what cannot be meditated upon.