Talking about Ghosts.... Sorry, I mean...
I've gotten quite into this, watching them all over a couple of times, and I've come to a bit of a fan theory; which I'd love to think is true... Alison can seem to only see Ghosts whom have been murdered...
That's my whole theory, here's the thinking.
Mary - Burned at the Stake
Thomas - Shot in a duel
Fanny - Thrown from a window
Pat - Shot with an arrow
Humphrey - Beheaded by accident, but whilst hiding from being wrongly accused of plotting against Queen Elizabeth I.
Then secondary ghosts, she see's:
Luftwaffe pilots - you could say were murdered, killed in action, they're still "mid-flight".
Civil War (Round Head) - appears to have been skewered by a pike.
Hitch-Hiker - Literally says she was murdered.
60's Architect - looked like he had been stabbed in the head as he waved from the window.
And the whole basement plague pit are all found to have been killed by Mick bringing the plague back with him.
So, murder or killed, each has a known perpetrator of their death.
This leaves a few holes in my theory, which I would like to plug...
Kitty - It does seem in Series 3, Episode 5, we have the lead up to her adoptive sister murdering her... We're yet to hear the full truth, but it seems to be the way that is played out.
The Captain - Well, he's a soldier, stationed at the hall we can't rule out a bomb or accident taking his life, he certainly was willing to sacrifice himself by throwing himself over an explosive device; and his love interest subordinate had been posted to the middle east, so there's something there to be explored.
Assuming the above this only leaves us Robin and Julian... Robin lived in pre-history, the stone age, a brutal time, his death may very well have been at the hands of another, be it animal or human; it does not seem likely with his describing wolves removing faces and eyes, and keeping his fathers skin, that he met his maker from his fur bed.
Julian is perhaps the biggest stretch for my theory, as we know a lot about his death, "the perfect death, booze, bit of rumpy pumpy and out".
But it wasn't alone, clearly the woman with whom me was in congress had a literally deadly effect on him, and he found the whole episode to be the most traumatic event in his life...
So there's my theory, the persons who remain are those who meet their end at the hands, or by cause, of another; and they can't resolve it.
The only two I can't answer this for are Annie the maid and the Ghost Doctor.... We shall see.