The problem with these subjects in that often the subtext is often not mentioned. These subtexts are the world views, starting points and premises of how people think the world works in combination with their own critical thinking and experiences. The subjective and objective worlds are at issue also in terms of personal and publicly or scientifically demonstrable experience. Also is the issue of the connections between the subjective and objective worlds.
It is of course true that people who have seen ghosts for instance like me, could have seen an entirely subjective thing that is caused by physical structures of the brain. I.e. what I saw is not there at all in the physical world but only in the internal virtual world of mentality. If this is case then it is caused by a mental illness of some kind, or some kind of disruption to the normal way the brain processes signals, as with the God helmet and perhaps certain meditational techniques that are known to also change the way the brain functions.
If it is a case of abnormal brain function that is responsible, then certain features in reasonableness will have to also be present. These would include there being no multiple witnesses to the perceived event, as what is seen has no objective or shared reality. If the phenomena of mass hysteria really is true, which is debatable, then for real cases of the paranormal like ghosts and so on, multiple witnesses would best be physically apart in order to overcome this objection. The problem with mass hysteria is that if true, then evidence for anything including science can also be suspect because there is doubt being placed on evidence itself, even if corroborated by multiple eyes. Another feature one would expect in the case of abnormal brain function is the complete absence of any physical effect in the objective world as a result of the perceived supernatural experience. Again if there were such effects then multiple witnesses would be needed, along with the correct types of circumstances in order to rule out fakery in the reporting of such cases.
It has to be acknowledged of course that in cases of no witnesses being present and mental illness being operative in the one witnessing such things, that this doesn’t of itself mean nothing paranormal happened. Even mentally ill people who hallucinate can also see or experience real things in the objective world. How about the subjective world? Can the subjective world of personal mentality be subject to other mentalities? This is an open question. Although there is little doubt that disrupted brain function can cause literal perceptions of subjective manifestations, or hallucinations for an individual, it is less clear what perception is anyway.
If one is a materialist as many, although not all atheists are, then perception is an allusion of the brain, when really what is occurring are the simple effects of particles on other particles, and that is all. If however the famous double slit experiment is true, then this view is suspect, for perception seems to be the very thing effecting the change in behaviour rather than particles effecting particles. In order to explain this in merely physical terms, the explanation can has to be kicked further down the road, as with a jigsaw with one piece permanently missing and another being moved to fill the gap on on-going basis.
Materialism is but one view of many and is certainly true to a degree, or science would not work, but it raises as many questions as it answers, not least how it itself can come to be in the first place. There is no way to prove that it is an exclusive explanation, and because of this other views exist.
Even science and materialisms foundation is fundamentally reliant on witness testimony, or more specifically multiple witness testimony, when one thinks about it carefully. There is another feature of science that is also part of the foundation, and that is comprehensibility. Does it fit into a matrix of known and understood casual relationships, to put it more technically? Here is should be mentioned that quantum mechanics of double slit fame does not fit into this criteria interestingly enough, as many experts, although not all, in that field will attest.
So comprehensibility and multiple witness testimony are keys to science and by extension materialism, which is the view that science is all there is, in terms of knowledge. However all witness testimony, even multiple witness testimony is based on subjective perception. Only an individual can agree or not with anther in essence. This being the case, what of comprehensibility? Well as stated before, there are problems with even this in what gets called science.
So perhaps it is not wise to so easily discount subjective perceptions as having some actual reality even with a lack of real world effect or multiple witnesses. However there are cases of the paranormal or supernatural that do have multiple witnesses who are not always in the same location and cases of real physical effects. These cannot be easily dismissed as mental or abnormal brain function nor should they. Of course, if one starts off with the view that such things are impossible because they think that science indicates materialism, and materialism indicates that anything that goes against comprehensibility is wrong, then perhaps this is the result of fear and insecurity that there may be more going on than they realise. There is a certain comfort in living in a box because what they cannot see can’t hurt them. This is a daft view to have, considering that there are issues with the box as good as it is, in the objective public world we live in, even though it gets mistaken for the totality of existence.
There is evidence that one’s own inner world is not entirety cut off from other people. It’s not just the double slit experiment that provides this but the phenomena of shared dreams. These can be verified. If this is the case, then even internal perceptions may relate to something `real` i.e another person’s mentality and perceptions. Even if no one else sees the ghost and it is not physically there, it doesn’t mean the ghost isn’t `real`.
Just a few thoughts.