Outer Monstrosities
Carnacki's opposition breaks down into 4 different categories;
Conventional 'ghosts' appear (though they are never referred to as such) as forces of inanimate human trauma materialised and made animate. They are referred to as either Saiitii and Aeiirii based on entries in the Sigsand Manuscript. (See Sigsand)
What are described as Ab-Human or Outer Monstrosities are malevolent entities, spirits perhaps, seeking to enter our world from outside. Though they resemble demons Carnacki is as ever keen to avoid any trace of religous explanation. His cosmic theory of their origin as in The Hog seems as desperate as it is unconvincing.
There is of course the entirely natural, 'Scooby Doo', earthly explanations, which we are told make up the vast majority of the sources of Carnacki's haunting. Such supernatural plots are usually dismissed as 'cheat endings' by film buffs but WHH is capable of making even these readable and entertaining and as several stories (such as The Find) never pretend to be anything but an entirely natural detective story the 'cheats' seem slightly easier to take.
Finally the supernatural explanation in Horse Of The Invisible is explained as 'Thought Induction' - unconconscious living psychic forces we might recognise as a poltergiest or a Monster of The ID.
Cosmology
According to Carnacki's theories human beings or "human-vibrations" exist in the material world but connected to the astraal via their soul. This sounds quite like standard contemporary Edwardian spiritualism. However in The Hog Carnacki theorises an exterrestrial place and source - the astraal world would seem to consist of a spinning Outer Circle (Aristotelian ?) of gas surrounding the earth within which the "Ab-human" and/or "Outer Monstrosities" thrive and evolve much like monsters of the deep in our own seas (WHH is perhaps best known for his fascination with horrors in and around the terrestrial oceans in many classic ghost stories of the sea) The Outer Circle is governed by unspecified 'Protective Forces' which strive to stop interferences by the "Ab-human" and/or "Outer Monstrosities"
The Protective Forces again suggest some religous source which Carnacki cannot bring himself to recognise.
Saiitii and Aeiirii
The Whistling Room provides a real case and cause for a Saiitii manifestation - "continuity of thought producing a positive action on the surrounding material" or later "a living spiritual fungus which invloves the very structure of the aether-fibre itself and, of course, in so doing acquires an essential control over the "material substance" in it'. Notice the lack of the words "ghost" and "haunting". In fact the nearest equivalent to the comparatively harmless ghost we know is the 'mere' Aeiirii manifestation mentioned in the same story.
Outer Monstrosities
Bad as they are Saiiti do seem to be of terrestrial origin. Harmful ghosts whereas the "ab Humans' and 'Outer Monstrosites' described in Gateway Of The Monster and The Hog are a level above - perhaps the eqvivalent of demons or devils.
For more detail see Forgotten
Futures - The Carnacki Cylinders
