Ok... so - step one - no taking the cop out answer and saying there are no ghosts. No, no. None of that.
Alright, now that that's out of the way.... assuming there is at least one ghost in the story, who is it?
Good lord, each possibility and combination just twists the story even more. There's a reason I don't get bored of this show. You see.... each performance I think up a different combination.
For example- today I pondered... alright, what happens if the ghosts are Flora and Mrs. Grose? And that's how I watched the show- it changed the whole damn thing! Every performance is new and twisted. Way too much fun.