As long as there have been reports of ghosts (forever), shouldn't there be some hard evidence of them by now?
If they can be seen, then shouldn't they make an impact on the physical world that can be measured and studied?
But on the other hand, I did have a pretty weird experience myself once that I can't explain. My dad's company was having a company picnic. Next to the field where we all were was a graveyard. As I was eating my burger and looking across to the graveyard, I noticed a man in a nice suit (possibly a tuxedo) walking around. I figured he was visiting a grave...no big deal. I didn't really think anything of it. But then the guy got to one grave and started to lay down. That was a little weird. The grave had cement on the top of it that made it look like two small steps. It looked like he laid down right next to it so that I couldn't see him anymore behind the steps. This was also weird because the "steps" didn't seem high enough off the ground to accomplish that.
Quite a while went by and I never saw him get up and curiosity took over. So I walked over there and saw that the "steps" were far too shallow to have fully hidden a full-grown man and there were no other holes or dips in the earth nearby.
I'm still extremely skeptical of ghosts for the reasons I posted above, but that was a pretty strange experience nonetheless. Until I see some good evidence for ghosts, I am going to assume there was a logical explanation for what I saw even though I don't know what it is.