Existence of Ghost

I came to my friend's place in Cirencester (Cool country side in GL, UK) and after the dinner our conversation started revolving around the ghosts.
My friend (Sam) was telling me that he has heard voices in one of the room in the house and knowing sam to be a sombre and courageous guy , unless he was intentionally trying to pull it on me (which is very unlikely) , i had to beleive what he told.

This again rose the curious mind inside me to kindle lot of questions in my mind, whether the ghosts really exist or is it a sensory perception?

The earliest ghost story that i have heard in my life was from my dad, who saw a person sitting on a restway bridge and didnt have any legs and that was a long story, my story being told by dad whom i respect for his analytical abilities ( i am still trying to outwit him without any success ) i couldnt argue more.

There are couple of insignificant encounters in my life where i came close to seeing a ghost but not quite successfully.
So what are the points for a ghost to exist, more than that what could be a definition of a ghost
Google says "def ghost" ghost as ,
"The spirit of a dead person, especially one believed to appear in bodily likeness to living persons or to haunt former habitats"

Ghost is a spirit of a dead person which will roam around their known places of hanging around.
So whats a spirit, Is it a collection of thoughts that represent the dead person? If it is a collection of thoughts, then whose definition it is? Ours or its(his/her)?

Is it just a phenomenon running in our confused minds? I strongly beleive so, unless you are told that the place you stay used to be a cemetry (we scared our collegue to hell by telling him so), your perception about the place is entirely different and the moment you hear that the place used to be a cemetry , your logical fixations start. So is a ghost just exist in our minds to attribute to the facts that we couldnt explain out of the normal sciences that we know? Most likely yes i would say, Our thoughts and conclusions are always based on the facts and fallacies of the science that we know and anything that we cannot decipher out of the science, we either reject or refuse to accept.

But it would be definetly interesting to understand whether the voices have been heard by more than a guy at the same time(provided nobody was walking around the closer road by) .