I live in a haunted part of the country, where traditions of ghostly visitations, headless horsemen and phantom coaches extend back to our primitive forefathers and foremothers. Moreover, there are plenty of moorland expanses where oddly shaped stones surround the unwary visitor and even sometimes threaten him.
I do not think I am an unduly fanciful person, but every now and then, while walking or cycling along one of our twisty lanes, I see dark figures retreating from the road into the shadow of the hedge, only to discover nobody when I come alongside. At a distance, a dark holly clump can easily convey the impression of someone lurking, and this is unnerving because lurkers always provoke the primitive aura of danger. Nowadays, individuals in motor cars are far more likely to lurk than those on their own two feet, because criminality goes hand in hand with sloth. Nevertheless, the impression remains powerful.
This brings me to the subject of hallucination and the meaning of that term. The psychologists define it as "an experience having the character of sense perception, but without relevant or adequate sensory stimulation". There was a time, some years ago, when I was followed by a small brown dog on my walks. All the family dogs had been black or white, so this made little sense. I could see it out of the corner of my eye, trotting happily along, and if I turned my head it was no longer there. The important part of the experience was that I knew beyond all doubt that it did not exist in my own particular time and space, but I continued to see it for many months. Since I was not deceived, was this hallucination, or was it not? Is there a better diagnosis?
I am aware of an associated problem which I find worrying. The existence of any material object depends on three dimensions in space and one in time. Without duration there can be nothing. Without space an object can have no location for its existence. Every space has at some time or another been occupied by something material and the only factor preventing two or more objects from occupying the same niche in space simultaneously is their staggering in time. Woe betide any matter which finds itself by some temporal defect trying to occupy a spatial niche at the same time as another. That would result in the explosive liberation of energy, and we should classify the event as an accident. If this were to occur, the energy liberated from the collision might manifest itself as a haunting.
T. C. Lethbridge in his book 'Ghost and ghoul' (1961) mentions a friend, a famous arachnologist not given to riotous imagination, who in the Shiant Islands in the Hebrides left his coat and lunch for a brief period on a rock and was never to see either again. The island had been completely uninhabited since it had supported a hermit centuries before, and the arachnologist attributed the disappearance to the mysterious fairy Sith reputed to exist in those parts. Did the event represent some disturbance of the space-time continuum whereby objects not only appear but also disappear?