but I no longer doubt the fact that thre are ghostly activities whatever causes them.
^This^, but for myself I lean towards an top-down interpretation (in the brain) of more mundane things that look enough like 'something' to be interpreted as 'something'. I've no doubt people have seen ghosts and they were real to them, but I'm very sceptical of photgraphs of the same. Although some photographs when deconstructed to illustrate quite nicely how the brain is designed to 'see' people and faces and will fill in the gaps in an extraordinary way.
Some ghosts I think are entirely visual reconstructions. A place might have a set of lighting conditions that can be briefly interpreted as a 'grey lady climbing the stairs' for example, and once the story is abroad each successive 'seer' interprets using the story of the 'grey lady climbing the stairs' as a baseline. It's suggestive that many ghosts are grey drab things that make no sound.
Some others I believe are triggered by smells, by odours we can't consciously detect but the
Jacobson's Organ picks up on and relays to the limbic brain. As many of these odours are bound up with hormones and 'other people', it seems a strong candidate for the triggering of interpretations of simulacra phenomena as phantoms, or on a lower level, a strong sense of not being alone, (often in a bad way).
(So for a 'grey lady' we might be looking for a chemical exuded by moulds or old wood or stonework that is suggestive of a female presence)
In then end, if someone truthfully tells you they saw a ghost, that
you can't see it or explain it, doesn't mean they didn't see it. However it doesn't mean it was an entity in it's own right either.
The latter point is the stumbling block for many as the frailty of our own senses, especially sight, is not something people want to confront. If their phantom wasn't real, what else isn't?