I do believe in them, though I have no memory of encountering one. Apparently, according to my parents, I became friends with one in Williamsburg, Virginia when I was 1 or 2. I was riding on my dad's back (in one of those baby slings that have the parent and baby back-to-back) and I was chattering happily to someone behind us. So, of course, my parents turn around to apologize to whoever I was bothering and realize that the only thing behind us at that height--my dad's 6'2"--was a portrait. Now, could I have been talking to the portrait? Sure, but I never did it again and when my mom asked if the mansion was haunted, the answer was yes. So who knows?
Now, anyone who knows me, knows that I'm a very logical person. Even though I write fantasy, I write it using scientific knowledge that I have gleaned from personal research as well as assumptions and "what ifs" on my part. For example, if I'm designing a vampire, I decide how that vampire would live off blood. Blood isn't very high in nutrition--it's mostly plasma and protein, as well as a lot of iron; there aren't a lot of carbohydrates. So, I debate with myself, what if instead of going through a complex digestive system that pulls the nutrients from the blood, the blood is swallowed and stored directly into the heart? So, by this logic, a vampire would have to drink its weight in blood every night to fill its enlarged and structurally altered heart, so that that blood could be pumped slowly through its body during the day. Also, because being tall puts a strain on the heart, most vampires would be short. So now, a) vampires have completely different body systems than humans and b) they have lower blood pressure, making them slow-moving and thus, strategic hunters. Now, if they're slow hunters, of course they'll hunt at night while their diurnal prey sleeps. What if they wake their prey up? I apply carnivorous plant logic--you give off either a scent or appearance that makes you attractive, or you use that scent or aura to lull them into a sense of security. You also want to target dull-witted prey that have a lot of blood to spare, so instead of preying on humans, vampires are more interested in cattle, which is actually the main prey of vampire bats. But humans of course, see the vampires as a threat, maybe because humans have been attacked in the past. Like sharks. Only a few people are attacked by sharks every year, but they still have the reputation of being maneaters. People are paranoid.
I kind of got off on a tangent there, but you see my point. Every assumption leads logically into the next and suddenly, the idea of vampires isn't so crazy. It fits quite of few of the legends (feed on blood, night creatures, smart, attractive) while still keeping something of a grounding in reality.
Now, getting back to ghosts, I personally think that we don't know enough about the topic to definitively say that ghosts exist one way or the other, but I do think that there is a lot of unexplained phenomena in this world that could be attributed to a "ghost". Is it definitely a ghost? Who knows? But, with the overwhelming evidence that there is
something going on that doesn't fit our "rules" of reality, why wouldn't I be open to the idea?
Just think about this, people used to think that sticking a leech or two on someone's arm to draw out the "bad humors" was a good idea.
-Toki