Can some people see ghosts?
At times some people claim to see a person, and even hear the person, when no one else can see what they are describing. How is this possible?
One possibility is that the eyes of those who see ghosts can see beyond the range of colors that ordinary eyes can see. That is, they can see infrared/ultraviolet. This case is easily refuted.
The entity (person, ghost) being seen is either transparent or not. If the entity is transparent, neither kind of eyes can see it. On the other hand, if the entity is not transparent, normal eyes will not see the entity, but they will see something dark instead.
Another possibility is that the entity is emitting some kind of energy, while transparent relative to light waves. For instance, it might emit radio waves in a range specific to ghosts. This requires the kind of eyes that see such energy to be different from ordinary eyes. This difference must be verifiable by examining the eyes. After all, for brain to sense something the eyes must have the means of providing the necessary signals. So, in this case perhaps someday we will discover the difference between the eyes of those who see ghosts, and ordinary eyes.
There is one other bizarre possibility. The entity may be transparent to visible light, but emit light in the range of infrared or ultraviolet. In that case, the same eyes that see ghosts should also be able to see in dark without wearing special glasses. This can be tested without examining the eyes.
If the tests mentioned here fail, and we find no difference between the eyes of those who see ghosts and our eyes, it will be safe to say no one can see ghosts. Whether there are ghosts was not the point of this note.