a) Do ghosts (i.e. the semi-transparent manifestations) exist? Probably. Whilst I see the logic in saying that "pink elephants covered in space ooze" are the same according to science, they're not, as many more people have reported seeing ghosts than have reported seeing pink elephants covered in space ooze.

b) Are ghosts souls/spirits of the deceased? Probably not, maybe they're some kinda electromagnetic concentration or something, fuck knows..

Anyway in conclusion I'm saying that ghosts are another unsolved phenomenon, in the same way that people may have once called Aurora Borealis angels dancing or something..