Image by Pete Linforth from Pixabay

First, let's distinguish between experiential and non-experiential ghosts.

Non-experiential ghosts are, lets say (for now), constituted of non-physical stuff, ectoplasm. They engage in stereotyped, repetitive actions (shaking shackles, gliding down hallways) but don't think and have no conscious experiences. The person whose living form they resemble is dead and gone with no personal psychic connection to the ghost.

Experiential ghosts, in contrast, think and have experiences, maybe engaging in more complex behavior. While there's something it's like to be an experiential ghost, there's nothing it's like to be a non-experiential ghost -- just as there's nothing it's like to be a mirror image or a shadow or a footprint. The following discussion is confined to non-experiential ghosts.

How do non-experiential ghosts come into being and how are they perceived? Here's a theory-sketch: A person (a projector) has a psychological trauma that creates a non-physical ectoplasmic entity resembling her. Once created, this ectoplasmic entity exists independently of the projector's experience. Such ghosts are seen and heard not by reflecting or creating photons or producing sonic vibrations in the air. Rather, they work directly on the perceiver's visual and auditory cortex. This direct action on the brain explains why ghosts cannot be photographed or audiotaped. Call this the ectoplasmic theory of ghosts.

Here's a competing theory -- the materialist theory. There is no ectoplasm. Rather, when a certain sort of trauma occurs, it directly affects the brains of ghost perceivers, through "paranormal" but perfectly physical action at a distance, cutting out the ectoplasmic middleman. A traumatic event in Schnerdfoot's brain as he is murdered directly causes visual and auditory cortical activity in the brains of other perceivers who walk by the scene of Schnerdfoot's death years later.

Now suppose, further, that the materialist theory of ghosts turns out to be true. It seems right to say, then, something like this: Ghosts are really nothing but effects on our brains from earlier trauma in other people's brains. There are no immaterial entities or properties (setting aside any qualms about whether consciousness itself might be immaterial).

Question: Could a philosopher in Materialist Ghost World run the following argument? I know that it's a law of nature that whenever there's a ghost it's produced by trauma of such-and-such a sort and has such-and-such effects on perceivers' brains. Yet I can conceive of those causes and effects without the presence of a ghost. Therefore, "being haunted" is not the same property as "being a place in which past trauma causes certain effects in perceivers' brains". There's a possible world in which those properties come apart. Furthermore, since I know Schnerdfoot's house is haunted, I know that materialism is false: There are non-physical properties instantiated in my world!

Since it seems wrong to say of the Materialist Ghost World that it is a world in which non-physical properties are instantiated, there must be some flaw in the argument. I've come to think it fails as a challenge. Here's why: Our Materialist Ghost World philosopher must, it seems, either conceive of ghosts ectoplasmically or conceive of them functionally (in terms of their causes and effects). If the first, it's false to say that he knows that ghosts exist in his world. If the second, it's false to say that there is a physically identical possible world that doesn't contain ghosts.

Eric Schwitzgebel  - http://schwitzsplinters.blogspot.com/2007/11/metaphysics-of-ghosts.html