"Learn to see yourselves as others see you...and you'll keep."
When Iris says to Lily, who she keeps calling Polly, "You pretty thing are all alike.....Learn to see yourselves as others see you...and you'll keep," I think Iris is talking about when the ghosts confine themselves to an area to try to see the last moments of their lives, instead of moving on. Confining themselves is how they rot, trying forever to look at themselves, instead of acknowledging that they're dead, and moving on.