It is interesting to see everyone's different perspectives on this. I am a believer of ghosts and that is because I experienced paranormal activities when I lived in a house I had moved into around 21 years ago now.
I remember the house I was in, I was laid in bed one night and I could hear a little girl crying for her mum, thinking it might have been a child outside upset, I went to check and there was nothing. A few nights later my then boyfriend could hear it but I couldn't.
We also had instances where we would see shadows on the wall near the stairs but no one was coming down the stairs and we had instances of things being moved and doors slamming with no given reason.