I've seen things before, once saw an old woman walking down a hallway for about 3 minutes when I was a child (she was see-through). And many, many more things have happended to me that others would contribute to ghosts or spirits. It's really amazing what your mind can make you see. I don't know what causes things like this to happen, but they do happen on occasion. I just know better than to think I'm seeing ghosties.
Also, I sort of believed in ghosts as a child and I do not now; the "happenings" have pretty much ceased. Except at night if I wake up from a dream, I'd see things moving around in my room, which could just be my eyes and light, or me still dreaming. What I have learned is that when you believe, even just a little, you tend to see, or notice, things more and contribute supernatual origins to them, but when you don't believe at all, it just doesn't happen or if it does, you know it isn't supernatural and you realize what other things it could be. Unless, of course, you have a psychological illness that causes you to see/hear things.