Ghosts: lost souls, a phenomenon, or just a fairy tale?

To be honest, I have a rather old-fashioned view on this matter – I believe that ghosts don't exist :D, and I won't believe in them until I encounter them myself. It's very easy to explain them as hallucinations, which are also amplified (in many cases) by the desire to see a ghost. But in reality, I certainly don't have complete certainty regarding ghosts...