What would constitute proof of a ghost?

[Mirrorglass]

But your point falls short. A material thing the size and shape of a human that can pass through walls is self contradictory.

And you haven't given me any example of a thing that is sometimes invisible, sometimes solid, etc. These things are self contradictory.

No, this is not correct. These things have never been observed, no, and they are extremely unlikely to occur. But they are not self-contradictory. A solid object that is invisible is theoretically possible (for example, by thermoptic camouflage), and so is an object the size of a human that at least appears to pass through walls while retaining it's shape. I can't think of a thing that could do that and be solid, but since human capacity for illusion is well known, appearing solid is hardly an impossibility.

No. All that matters is whether a ghost exists. Some unexplained phenomenon is not necessarily a ghost.

It is if that's the way we define 'ghost'. And that's what I've been doing here; defining ghosts as a phenomenon satisfying certain conditions, unexplained and unexplainable by trickery or illusion. Other definitions exist, sure, but I think this is the definition one should go by, if one wishes to seriously and honestly research the phenomenon.

You're kidding, right? You want examples of purported ghosts that were said to walk, to make sound, to be photographed, etc.? All of these things are logically inconsistent with passing through a wall.

Claims by other people don't really matter to the definition I'm making. Maybe those ghost sightings were fake, or the people were mistaken? Or maybe the ghost just appeared to walk, while actually just floating and moving it's legs?

And again, it's theoretically possible for a thing to be able to make sound, be photographed and pass through walls, and quite possible to appear to do these things.

Again, not without defining the term. (By the way, how does writing it down make the problem of information leakage go away?) Why couldn't the result be due to telepathy? Are you defining ghosts as being telepathic, but no human can have that ability?

There's no more need to rule out telepathy here than in any other examination. Until telepathy has been shown to exist, we'll just assume it affects our results no more than the tooth fairy does.

If by "phenomenon" you mean something that is observed, then yes, but that's a tautology. One can also be self deceived (with no trickery involved), as for example optical illusions, pareidolia, etc. So elminating fakery doesn't prove anything except no fakery.

[ETA: I could add faulty memory, delusions, hallucinations, etc., but I was thinking primarily of self-deception along the lines of the Clever Hans story. There was no trickery and people observed the "phenomenon" of a horse apparently doing arithmetic. It turned out that the horse was not doing arithmetic. I think many ghost hunters deceive themselves.]

Fair enough, let's rule out illusions as well.

And such a definition is logically inconsistent. If it can be detected, can produce sounds, is human-like and at least sometimes visible, then it can't possibly "not appear to have a solid material existence". These concepts are logically inconsistent.

It seems you're defining 'not appear to have a solid material existence' differently from me. No matter; let's just replace the phrase entirely with 'floats around, occasionally becomes invisible and/or passes through walls'. That should prevent further confusion.

Also a thing can not be visible and invisible, or audible only to some people. (That's not how light and sound work.) As for floating around, that by itself would not be sufficient to distinguish a ghost from an otherwise normal human with the ability to levitate.

I already addressed the invisibility thing; there's no logical problem. I never said anything about being audible to only some people, but that isn't impossible either.

This was more in response to those who refuse to attempt what you're attempting: that is, offer a formal definition of the term ghost. They said that everyone knows what ghost means and we should just go with the conventional usage. I'm pointing out that that conventional usage is also logically inconsistent.

Yes, I'm also not too fond of people who claim 'everybody knows what a ghost' is as an excuse not to define it. But I still think you're wrong about the logical inconsistency. Most definitions of 'ghost' are very unlikely to be true, but not logically impossible.

 

[Nursedan]

Sweetie, a thread is like a baby bird. You can't own it, you just start it off and let if free. After that it will fly where it wills. If your question was answered, you have no obligation to keep posting here, but the conversation will continue as long as there are two people who hate each other disagree. So don't worry about it, the thread will die when it's time has come.

Ha!! I love this. Thanks for the smile.

Yeah, I know it's not up to me to dictate the direction of the thread. I just want to make clear that I "get it".

 

[EHocking]

If Houdini appeared to James Randi and exposed a secret that only Houdini could have known, I would give the thought of ghost another critical look. That would be a good start point though.

I think it could only constitute the message that Houdini said he'd convey post-mortem to any medium that contacted him.

A magician exposing another magician's secret?
Not irrefutable.

While I might be inclined to trust James Randi at his word, but without something irrefutable, all we'd have is (another) anecdote and/or argument from authority.

 

[JoeTheJuggler]

So by definition we can't define what a ghost is, because a ghost is logically inconsistent.

That's not "by definition" but by the failure of anyone to provide a formal definition of ghost that is complete and logically consistent.

A bird can't live underwater, but some birds can swim. A fish can't glide for hours on thermals, but some fish can fly (a bit). They're all classed as living animals, though; just because we can't find one single organism that can both live underwater and take to the wing doesn't mean the other types can't exist. I don't particularly see why you shouldn't have different types of ghosts.

We can give a logical definition of the term fish that includes what we mean and excludes what we don't mean. We can do the same for individual species. Apparently we can't do that for the term ghost just as we can't do that for a 4 sided triangle.

Furthermore, I think that arguing "a ghost is logically impossible therefore we can't specify what would convince us as a ghost" is especially satisfying.

Your sense of satisfaction is irrelevant to whether this is true or not. If something is logically impossible then there is nothing that would constitute proof of its existence. If the term is undefined, then asking what would constitute proof is meaningless. This is not a difficult concept to grasp.

Surely there must be something that would be utterly convincing?

Convincing of what? Without defining the term ghost, we don't know what we have to be convinced of. We don't know what conclusion we would need to reach deductively or what measurement we would have to get in an inductive approach.

 

[JoeTheJuggler]

No, this is not correct. These things have never been observed, no, and they are extremely unlikely to occur. But they are not self-contradictory.

Both visible and invisible is a logical contradiction. Both "seems to be immaterial" and "able to be seen, heard and recorded" are logically contradictory. This isn't a matter of probability. These are logical contradictions.

 

[ksbluesfan]

I think it could only constitute the message that Houdini said he'd convey post-mortem to any medium that contacted him.

A magician exposing another magician's secret?
Not irrefutable.

While I might be inclined to trust James Randi at his word, but without something irrefutable, all we'd have is (another) anecdote and/or argument from authority.

That wouldn't constitute proof, but it would make me curious enough to look into it further.

 

[JoeTheJuggler]

It is if that's the way we define 'ghost'. And that's what I've been doing here; defining ghosts as a phenomenon satisfying certain conditions, unexplained and unexplainable by trickery or illusion. Other definitions exist, sure, but I think this is the definition one should go by, if one wishes to seriously and honestly research the phenomenon.

You're saying that the words paranormal and ghost are interchangeable? If so, then I would agree it's possible to say what would constitute proof (inductively) of the paranormal. I think that "definition" of the term ghost is so far off of the conventional one that it's an abuse of language.

It would be similar to giving an operational definition of the construct intelligence to be equivalent to height. If you said that's what it means, then it would be a simple matter to know what to measure (height), but I think it would be an abuse of the language to say that height and intelligence are the same thing.

It seems you're defining 'not appear to have a solid material existence' differently from me.

I'm using the words in the ordinary way. If something can be seen and heard and photographed, it does not appear to have no material existence. That is a logical contradiction.

I already addressed the invisibility thing; there's no logical problem. I never said anything about being audible to only some people, but that isn't impossible either.

There's no logical problem with a thing having the characteristics of being both visible and invisible? Of being able to produce sound and not being able to produce sound? This is the very epitome of logical contradiction.

Most definitions of 'ghost' are very unlikely to be true, but not logically impossible.

That's begging the question. I've yet to see a formal logical definition that doesn't present a contradiction.

There's no more need to rule out telepathy here than in any other examination. Until telepathy has been shown to exist, we'll just assume it affects our results no more than the tooth fairy does.

I disagree. If you make the assumption that paranormal explanations are out, you can't allow in a paranormal hypothesis. I have no problem with making that assumption in real world inductive tests (drug testing, for example). But you can't make the assumption that paranormal explanations are out while you're attempting to test a paranormal hypothesis. That's just special pleading. (Either that or, as mentioned, you're equating ghost with paranormal, and the terms are not interchangeable.)

 

[JoeTheJuggler]

That wouldn't constitute proof, but it would make me curious enough to look into it further.

I agree.

It would suffice to win the Million Dollar Paranormal Challenge, but would not prove the existence of a ghost (especially without defining the term).

 

[Jeff C]

... open your mind... ya could have an experience tonight.. if you so desired...

 

[richardm]

We can give a logical definition of the term fish that includes what we mean and excludes what we don't mean. We can do the same for individual species. Apparently we can't do that for the term ghost just as we can't do that for a 4 sided triangle.

It's easy to define a 4 sided triangle. It is a triangle with four sides

I think we can come up with a definition of what a ghost is; or rather we can have a number of different definitions for them. We have things shaped like people who appear and repeat the same actions again and again; we have things that have no form and hurl things around; we have things that clank chains and promise you eternal damnation unless you repent of your ways, and so on and so on. These are what are commonly called "ghosts". Why is that not enough to work on?

Take one of those: Poltergeists. They don't have a visible physical form, but they chuck things around. Why is that not a useful working definition of a ghost? You seem to be asserting that it is not useful simply because by normal expectations things that are invisible are unable to chuck things around therefore poltergeists do not exist QED. It may not be a very robust definition and what is doing the chucking may not actually be a ghost - but then isn't that what the OP was asking? What would be incontrovertible proof of a ghost? Maybe invisible things that chuck things around wouldn't be - so then what would?

If something is logically impossible then there is nothing that would constitute proof of its existence. If the term is undefined, then asking what would constitute proof is meaningless. This is not a difficult concept to grasp.

I simply think you are requiring too rigorous a definition. I don't think the OP was asking for a formal proof of ghosts, just asking what criteria something would have to provide in order to be deemed a certain ghost. It could have been an interesting discussion about what isn't a ghost, apart from anything else.

 

[Beth]

But your point falls short. A material thing the size and shape of a human that can pass through walls is self contradictory.

Not necessary. It depends on what the wall and the ghost are composed of. For example, sound waves can pass though most types of walls.

And you haven't given me any example of a thing that is sometimes invisible, sometimes solid, etc. These things are self contradictory.

Water (or more accurately H20) is sometimes invisible, sometimes solid, etc. Such descriptions of water are not self contradictory.

 

[Resume]

Not necessary. It depends on what the wall and the ghost are composed of. For example, sound waves can pass though most types of walls. Water (or more accurately H20) is sometimes invisible, sometimes solid, etc. Such descriptions of water are not self contradictory.

If we're going to be fussy here, wouldn't it be more precise to say that water (H2O) is not invisible but varies in degrees of transparency?

 

[Mirrorglass]

Both visible and invisible is a logical contradiction. Both "seems to be immaterial" and "able to be seen, heard and recorded" are logically contradictory. This isn't a matter of probability. These are logical contradictions.

I think I see the disconnect now. You are assuming the ghost would be both invisible and visible simultaneously. That would be a contradiction, of course, but that wasn't in the definition; the definition stated a ghost occasionally becomes invisible.

You're saying that the words paranormal and ghost are interchangeable? If so, then I would agree it's possible to say what would constitute proof (inductively) of the paranormal. I think that "definition" of the term ghost is so far off of the conventional one that it's an abuse of language.

It would be similar to giving an operational definition of the construct intelligence to be equivalent to height. If you said that's what it means, then it would be a simple matter to know what to measure (height), but I think it would be an abuse of the language to say that height and intelligence are the same thing.

No, not really. Would you say that all paranormal phenomena include a ghostly human-like apparition? What I am saying is that 'ghost' and 'paranormal phenomenon that appears to be a ghost' are interchangeable.

I'm using the words in the ordinary way. If something can be seen and heard and photographed, it does not appear to have no material existence. That is a logical contradiction.

Do you notice that you once again dropped the word 'solid' from the sentence? That's actually a key point. What was meant by 'not appearing to have a solid material existence' was not 'is completely undetectable by any method', as you seem to believe. The intended meaning was along the lines of 'apparently behaves in a way different from solid objects'. That you misunderstood the point is understandable, but does not create a contradiction to the point itself.

I already addressed the invisibility thing; there's no logical problem. I never said anything about being audible to only some people, but that isn't impossible either.

There's no logical problem with a thing having the characteristics of being both visible and invisible? Of being able to produce sound and not being able to produce sound? This is the very epitome of logical contradiction.

Again, no one has stated the thing must be both visible and invisible at the same time. There's no logical contradiction in shifting between those states. As for 'being able to produce sound and not being able to produce sound', that's so far from what I actually said I'm inclined to call it a strawman. Although there's no contradiction in being able to produce sound at times and not at other times, either.

That's begging the question. I've yet to see a formal logical definition that doesn't present a contradiction.

No, it's summing up my argument. There's a difference. And I've yet to see a contradiction in my definition that isn't the result of you misreading it.

I disagree. If you make the assumption that paranormal explanations are out, you can't allow in a paranormal hypothesis. I have no problem with making that assumption in real world inductive tests (drug testing, for example). But you can't make the assumption that paranormal explanations are out while you're attempting to test a paranormal hypothesis. That's just special pleading. (Either that or, as mentioned, you're equating ghost with paranormal, and the terms are not interchangeable.)

See above for the difference I make between 'ghost' and 'paranormal'.

For the rest.. well, it doesn't really work. You're saying that if we test for a paranormal claim, then we can't assume other paranormal explanations are false, and thus can never prove the claim? That we must always consider ghosts and telepathy to be equally likely possibilities? No, that's not the way science works.

In the hypothetical scenario where a ghost correctly gave a number no human in the room could have known, that would be a real world test. Obviously we wouldn't start rummaging the room for hidden telepaths and leprechauns or blame the results on an invisible dragon astral projecting. Since the theory 'it was a ghost' is the simplest way of explaining the phenomenon, that would be considered true until new evidence suggested otherwise. No, we wouldn't have proven it wasn't the work of a mischievous telepath, any more than we've proven gravity isn't just a joke played on us by telekinetics. But it doesn't matter.

If you claim ghost sightings may be explained by telepathy, then it's up to you to prove telepathy. Paranormal claims aren't one, huge entity; they're all separate claims, and proving one would have no bearing whatsoever on any of the others. If a 'paranormal' claim turned out to be true, then that would just mean that particular claim wasn't paranormal after all, not that we must accept all paranormal claims as true.

 

[Mirrorglass]

If we're going to be fussy here, wouldn't it be more precise to say that water (H2O) is not invisible but varies in degrees of transparency?

Oh, please don't start questioning the definition of 'invisible' as well. We've too many ambiguous terms on the table already.

 

[JoeTheJuggler]

Not necessary. It depends on what the wall and the ghost are composed of. For example, sound waves can pass though most types of walls.

Sound waves are waves of minute differences in air pressure. There is no thing that penetrates a wall. Not one molecule of air penetrates a wall when a wall transmits sound.

ETA: similarly, when an electrical current propagates along the copper wires in my walls, copper wire doesn't come streaming out of my electrical outlets. The wire stays put. If you oscillate a string to make a wave travel along it, the end of the string stays in your hand and does not travel to the far end.

Water (or more accurately H20) is sometimes invisible, sometimes solid, etc. Such descriptions of water are not self contradictory.

This is the fallacy of equivocation or ambiguity. You're using the terms "invisible" and "solid" in different ways than the definition does. In terms of the definition, water is always solid and always visible.

If a "ghost" were like water in these terms, it would be easy to say what constitutes proof of its existence.

 

[JoeTheJuggler]

Oh, please don't start questioning the definition of 'invisible' as well. We've too many ambiguous terms on the table already.

Sorry--but Beth misused the term in this way. You can't avoid this by claiming that everyone knows what invisible and solid mean as well. Apparently everyone doesn't use the same meanings of these words.

 

[JoeTheJuggler]

I think I see the disconnect now. You are assuming the ghost would be both invisible and visible simultaneously. That would be a contradiction, of course, but that wasn't in the definition; the definition stated a ghost occasionally becomes invisible.

No, I'm assuming visibility/invisibility is an intrinsic characteristic. Any thing you speak of as being sometimes visible and sometimes invisible really isn't. It's just equivocation of the terms. When you speak of a ghost being invisible, you're not talking about it being a particle that's simply too small to see with the naked eye.

http://www.fallacyfiles.org/ambiguit.html

No, not really. Would you say that all paranormal phenomena include a ghostly human-like apparition? What I am saying is that 'ghost' and 'paranormal phenomenon that appears to be a ghost' are interchangeable.

But that's not helpful in the least. For example, imagine a fakir who has the ability to turn invisible, to levitate and to pass through walls. Is he necessarily a ghost?

Do you notice that you once again dropped the word 'solid' from the sentence? That's actually a key point. What was meant by 'not appearing to have a solid material existence' was not 'is completely undetectable by any method', as you seem to believe.

Then you're misusing the language. A thing that is detectable does not even appear to be immaterial.

The intended meaning was along the lines of 'apparently behaves in a way different from solid objects'. That you misunderstood the point is understandable, but does not create a contradiction to the point itself.

You're just trying to say it has the property of having the properties of something immaterial but is material in order to try to avoid a logical contradiction. But it is, nonetheless, a logical contradiction. It cannot have properties of seeming to be immaterial and seeming to be material. That is a contradiction. It cannot be both green and not-green.

Again, no one has stated the thing must be both visible and invisible at the same time. There's no logical contradiction in shifting between those states. As for 'being able to produce sound and not being able to produce sound', that's so far from what I actually said I'm inclined to call it a strawman. Although there's no contradiction in being able to produce sound at times and not at other times, either.

Addressed already. This is simply ambiguity with the terms visible and invisible. A ghost is not invisible the way a paramecium is "invisible". In fact, a paramecium is visible, but not to the naked eye.

No, it's summing up my argument. There's a difference. And I've yet to see a contradiction in my definition that isn't the result of you misreading it.

I've shown that you have failed to come up with a formal logical definition that doesn't have internal contradictions.

I'll concede that you have come up with an operational definition, but only by ignoring a great many parts of conventional usage of the term ghost. (Again, it would be like giving an operational definition of intelligence as a measure of height.) If you're content with that, then you'd have to accept that a fakir capable of those things is a "ghost". That's not at all how the term is conventionally used.

For the rest.. well, it doesn't really work. You're saying that if we test for a paranormal claim, then we can't assume other paranormal explanations are false, and thus can never prove the claim? That we must always consider ghosts and telepathy to be equally likely possibilities? No, that's not the way science works.

You've turned this around on yourself. You can't do special pleading. If you make a rule that says rule out paranormal explanations, you can't then proceed to test a hypothesis that is a paranormal explanation. (This is what you're proposing when you say you wish to exclude telepathy, or fakirs with the abilities I have described), and allow only the paranormal explanation of "ghost".

As I said, I'm fine with the rule of excluding paranormal explanations (Occam's Razor says you shouldn't create unnecessary entities), but if you use such a rule, you can't make exceptions to it, and thus you can't test ANY paranormal explanation (including the ghost hypothesis).

In the hypothetical scenario where a ghost correctly gave a number no human in the room could have known, that would be a real world test. Obviously we wouldn't start rummaging the room for hidden telepaths and leprechauns or blame the results on an invisible dragon astral projecting.

Or identify who is a fakir with paranormal powers and not a ghost. Yes, this is exactly why the approach you're suggesting fails to prove the existence of a ghost. ETA: It doesn't distinguish between a ghost and any of a myriad other paranormal hypotheses.

Since the theory 'it was a ghost' is the simplest way of explaining the phenomenon, that would be considered true until new evidence suggested otherwise.

Says who?

If you claim ghost sightings may be explained by telepathy, then it's up to you to prove telepathy.

I have made no such claim. I'm pointing out to you a flaw in your proposed method to prove the existence of a ghost. Even if you got the results that you think would point to a ghost, lacking a definition that can distinguish a ghost from other paranormal phenomena, you have failed to prove the existence of a ghost. I've been saying this all along. You could win the MDC with such a demonstration because you will have proven something paranormal, but you will have failed to prove the existence of a ghost.

Paranormal claims aren't one, huge entity; they're all separate claims, and proving one would have no bearing whatsoever on any of the others.

This is exactly the point I'm making. You are essentially saying that if you can prove the existence of something paranormal you have proven the existence of a ghost. It's not so because ghost claims are only a subset of paranormal claims. They are not interchangeable terms.

 

[Beth]

Not necessary. It depends on what the wall and the ghost are composed of. For example, sound waves can pass though most types of walls.

Sound waves are waves of minute differences in air pressure. There is no thing that penetrates a wall. Not one molecule of air penetrates a wall when a wall transmits sound.

ETA: similarly, when an electrical current propagates along the copper wires in my walls, copper wire doesn't come streaming out of my electrical outlets. The wire stays put. If you oscillate a string to make a wave travel along it, the end of the string stays in your hand and does not travel to the far end.

So do you consider 'sound' to be material or immaterial then? Is an electrical current material or immaterial? I thought those things counted as material by your definition.

At any rate, it doesn't strike me as much of an objection. Knowing nothing about ghosts or what they are made of, I don't consider it any more unreasonable to claim that ghosts can pass through solid objects like walls simply because humans cannot. Ghosts may be more akin to a sound wave or an electrical current than a human being.

Water (or more accurately H20) is sometimes invisible, sometimes solid, etc. Such descriptions of water are not self contradictory.

This is the fallacy of equivocation or ambiguity. You're using the terms "invisible" and "solid" in different ways than the definition does. In terms of the definition, water is always solid and always visible.

I don't think that people who claim ghosts exist and are sometimes invisible are using your definitions. I think they are using the term 'invisible' in the sense that sometimes ghosts cannot be seen. Just as water sometimes cannot be seen. So when you asked for something that, like a ghost, can be sometimes solid and sometimes not, sometimes visible and sometimes not, it seemed more appropriate to respond using the same definitions that people claiming ghosts exist are using. You can't claim they are impossible and self-contradictory when you are using different definitions of those words than were intended by the description.

If a "ghost" were like water in these terms, it would be easy to say what constitutes proof of its existence.

Yes.

 

[JoeTheJuggler]

So do you consider 'sound' to be material or immaterial then? Is an electrical current material or immaterial?

See my previous answer. Sound is a wave that propagates through a medium (air).

At any rate, it doesn't strike me as much of an objection. Knowing nothing about ghosts or what they are made of, I don't consider it any more unreasonable to claim that ghosts can pass through solid objects like walls simply because humans cannot. Ghosts may be more akin to a sound wave or an electrical current than a human being.

You misunderstand my objection. It's not that the property is something humans can't do, but that it presents a logical contradiction with other properties.

I don't think that people who claim ghosts exist and are sometimes invisible are using your definitions. I think they are using the term 'invisible' in the sense that sometimes ghosts cannot be seen. Just as water sometimes cannot be seen.

I think you're completely wrong. I think they mean ghosts are invisible by nature--that they can dematerialize. Not that they are translucent or too small to be seen with the naked eye.

You can't claim they are impossible and self-contradictory when you are using different definitions of those words than were intended by the description. Yes.

Seriously, when someone says a ghost is sometimes solid they don't mean that when it's not solid it's either a liquid or a vapor. I think they mean that it is sometimes material (and all 3 states of water are material--ice, steam and water) and sometimes immaterial (which is a logical contradiction). I think you're the one equivocating with the terms invisible and solid.

 

[Mirrorglass]

No, I'm assuming visibility/invisibility is an intrinsic characteristic. Any thing you speak of as being sometimes visible and sometimes invisible really isn't. It's just equivocation of the terms. When you speak of a ghost being invisible, you're not talking about it being a particle that's simply too small to see with the naked eye.

http://www.fallacyfiles.org/ambiguit.html

I really don't understand how you get to this conclusion. Why is it logically inconsistent for a thing to be visible at times and invisible at others? I've already provided an example of a thing that's just that, a man wearing thermoptic camuflage. Are you saying that this is logically impossible, or just that this man would never be invisible, despite being optically indistinguishable from his background?

But that's not helpful in the least. For example, imagine a fakir who has the ability to turn invisible, to levitate and to pass through walls. Is he necessarily a ghost?

If I were to investigate ghosts, then I would be open to the possibility that ghost sightings are actually fakirs levitating around. If that turned out to be the case, then that would be what we commonly refer to as a ghost - it would just mean that the theory about them being souls of the dead was wrong.

Then you're misusing the language. A thing that is detectable does not even appear to be immaterial.

You're using far too narrow definitions. It's perfectly okay to say 'a ghost is immaterial' when what you mean is 'a ghost can pass through walls and is only visible at times'. Most people will understand what is meant from the context. Words can have more than one interpretation, and it's the intended interpretation that matters, as long as it's fairly commonly accepted - and this one is.

You're just trying to say it has the property of having the properties of something immaterial but is material in order to try to avoid a logical contradiction. But it is, nonetheless, a logical contradiction. It cannot have properties of seeming to be immaterial and seeming to be material. That is a contradiction. It cannot be both green and not-green.

Yet it can be green now and red once I've painted it. And yes, a thing can have the property of seeming to be immaterial in some aspects and material in others. You're simply using terms far too strictly, and I don't understand why. Surely you don't really believe a word always means the same thing, despite the context in which it was spoken?

Addressed already. This is simply ambiguity with the terms visible and invisible. A ghost is not invisible the way a paramecium is "invisible". In fact, a paramecium is visible, but not to the naked eye.

Well, perhaps a ghost is visible to some instruments? It's perfectly acceptable to say something is invisible if it can't be seen by the naked eye, even if it can be detected via other methods.

I've shown that you have failed to come up with a formal logical definition that doesn't have internal contradictions.

No, you keep shoehorning those contradictions in by interpreting the wording of the definition in stange, overly literal ways. I'm still not certain whether you're truly confused or just being obtuse.

I'll concede that you have come up with an operational definition, but only by ignoring a great many parts of conventional usage of the term ghost. (Again, it would be like giving an operational definition of intelligence as a measure of height.) If you're content with that, then you'd have to accept that a fakir capable of those things is a "ghost". That's not at all how the term is conventionally used.

Yes, obviously some people use the term differently. That doesn't matter at all, though. The very point of this exercise was to construct a definition of 'ghost' we could actually test. We wouldn't have started if one of the common definitions did the trick. And yes, obviously by accepting my definition, one accepts we do not know what the root cause of the phenomenon is - and that essentially means it could just be one of those pesky fakirs.

You've turned this around on yourself. You can't do special pleading. If you make a rule that says rule out paranormal explanations, you can't then proceed to test a hypothesis that is a paranormal explanation. (This is what you're proposing when you say you wish to exclude telepathy, or fakirs with the abilities I have described), and allow only the paranormal explanation of "ghost".

As I said, I'm fine with the rule of excluding paranormal explanations (Occam's Razor says you shouldn't create unnecessary entities), but if you use such a rule, you can't make exceptions to it, and thus you can't test ANY paranormal explanation (including the ghost hypothesis).

Or identify who is a fakir with paranormal powers and not a ghost. Yes, this is exactly why the approach you're suggesting fails to prove the existence of a ghost. ETA: It doesn't distinguish between a ghost and any of a myriad other paranormal hypotheses.

No. That's just completely wrong. A hypothesis is considered paranormal if we believe it to be impossible in our world. However, if a hypothesis is shown to be true, then that means it wasn't a paranormal hypothesis. Thus, should I present proof of telekinesis, it would mean telekinesis isn't paranormal. There'd be no need to speculate other paranormal explanations any more than there is in normal science.

Of course, we would continue on to explore then phenomenon, and eventually find it's origin. That origin might even be a fakir - just as it's possible that we'll one day find out the sun is actually a fakir emitting heat. But until we do, we bloody well are going to call the Sun the Sun, telekinesis telekinesis and a ghost a ghost.

Says who?

Says everybody. That's how words work. We find out something exist, and we call that something by a name. If we later find out more about the nature of that something, the thing will still be the same thing.

For example, we know there's such a thing as quarks. We're not completely certain what quarks are made of (though of course there are several good theories). It's theoretically possible that quarks turn out to be very small fakirs. Would that mean they were never quarks in the first place? Of course not. And that's the same for ghosts. If we find a thing that satisfies the definition of a ghost, then it's a ghost, whether it's made out of atoms, fakirs or spam.

I have made no such claim. I'm pointing out to you a flaw in your proposed method to prove the existence of a ghost. Even if you got the results that you think would point to a ghost, lacking a definition that can distinguish a ghost from other paranormal phenomena, you have failed to prove the existence of a ghost. I've been saying this all along. You could win the MDC with such a demonstration because you will have proven something paranormal, but you will have failed to prove the existence of a ghost.

This is exactly the point I'm making. You are essentially saying that if you can prove the existence of something paranormal you have proven the existence of a ghost. It's not so because ghost claims are only a subset of paranormal claims. They are not interchangeable terms.

Again, no. I hope you understood the above explanations why. If not, I doubt there's much more either of us can learn from this conversation.