Back in 1982 or 1983 (I don't remember which), I was sitting on my bed reading, with the dog lying down at my feet. The dog suddenly started to growl and look out the open door into the hallway. I put the book down and looked out, but didn't see anything. Then, I saw what for all intents and purposes was a "transparent person" walk past my door down the hallway. The image was about 6' tall, male, with long hair and a long beard, kind of hunched, with lots of wispy smoky stuff around him. I couldn't make out any clothing, just wispy stuff. I could "see through" him, as though he weren't solid. He didn't look at me or acknowledge me in any way, just walked on by. The dog got up and went out into the hallway, continuing to growl, with all his hair standing on end. I got up and followed the dog, but didn't see anything, so I went down the hall into the kitchen and there was no one there. The sliding door to the backyard was still locked, so I know a person didn't unlock it and go outside. I felt a little odd, but the dog was quite upset for a few minutes and I had to "talk him down" to get him to mellow out. No one else was home, just me and the dog.
I've thought about this experience a lot, and here are some possible explanations:
The dog was upset about something (perhaps a noise, or a scent, or a dust mote), and in the absence of anything tangible, my brain filled in the gaps and created the "apparition."
Perhaps some sunlight got reflected off a car window out on the street, and was beamed into the house through the living room window, I saw the image on the wall, and concurrent with the dog getting upset about something, and again my brain filled in the gaps.
I hallucinated the whole thing, including the dog being upset. Likely? No, but possible nonetheless.
Or, it was something else which I can't rationally explain, given what I know about how things work. All three of the explanations I listed above rely on my having some kind of sensory lapse or sensory embellishment to be plausible. But what if I really did perceive accurately what happened that day? That's the notion that sticks for me, and is like the chink in the wall where the wind gets in.
I put the existence of ghosts in the same category as UFO's or visiting aliens, or el chupacabra, or sasquatch. I don't believe in them, because I have no evidence to support that belief. But -- I'm also not arrogant enough to state that definitively, they do not exist. I do not, for an instant, believe that we have it "all figured out." After all, at one time, the existence of microbes was considered preposterous, and people who advocated for their existence were ridiculed and discredited. If someone claims to have seen a ghost, I don't take their account at face value, but I also don't try and explain away what they claim either. I'm ok with there being some uncertainty and/or mystery in life, and I also think we've only even seen the very tip of the iceberg when it comes to scientific understanding of the world around us.
I work with cancer patients and cancer survivors in my day job, and fairly often, I encounter people who wholeheartedly believe that it was their religious faith and devotion/prayers that put them into remission. I don't see it that way, and I could try and explain that away for them, but that would be cheap and mean -- kind of like telling kids there's no Santa Claus. If holding onto that belief helps them in some way, perhaps it should just be left alone.
So when someone recounts their "ghost sighting" experience, I remain "open-mindedly skeptical." Maybe they saw a ghost, maybe they didn't, and it's not for me to judge. And maybe, just like with my cancer patients, holding that belief does something for them that I can't explain and will never fully understand.