"Grey Matters" is the 10th episode of the second season of the American science fiction drama television series Fringe. The episode was written by Ashley Edward Miller and Zack Stentz, and directed by Jeannot Szwarc. It centered on three mental patients who mysteriously became sane again after shapeshifters from the parallel universe removed a piece of foreign tissue from each of their brains; this tissue is later revealed to have been taken from the brain of Walter Bishop years before.