In considering this subject with the attention that is due to it, it has appeared to me that all the stories of ghosts and super, or, un-natural appearances, may be referred to some of the following causes:

1. To the augmentation produced by fear in any effect on the senses—thus the ear of a terrified man will convert the smallest noise into the report of thunder, or his eye will change the stump of a tree into a monster twenty feet high. As the senses are furnished for protection, their irritability, under the impression of fear, is part of their economy, as the means of preserving our being; but it is absurd to refer back the effects thus augmented, to external causes which might be capable of producing the augmentation. To such an error of the senses and of reasoning, is, however, to be referred half the ghosts and supernaturals of which we hear in village ale-houses, in nurseries and schools.

2. To diseased organs of sensation; as an inflamed eye producing the effect of flashes of light in the dark, or fulness of blood producing a ringing or singing in the ears. Sometimes diseases of the visual organs are accompanied by hallucinations of mind; and persons ill in fevers often see successions of figures and objects flit before their eyes till the disease has been removed. The workings of conscience or nervous affections will also produce diseases of the senses, and such hallucinations of mind as to occasion a person to fancy he sees another, or to be haunted by him. But there is nothing supernatural in all this; it is sometimes a local disease, sometimes an effect of fever, sometimes a nervous affection, and sometimes partial insanity.

3. To natural causes not understood by the parties. Thus, anciently the northern lights were mistaken for armies fighting; meteors and comets for flaming swords, portending destruction or pestilence; the electrified points of swords to the favour of heaven; the motions of the  planets to attractive effluvia; and all the effects of the comixture of the gases to benign or diabolical agency, as they happened to produce on the parties good or evil. So in the like manner old houses are generally said to be haunted, owing to the noises which arise from the cracking and yielding of their walls and timbers, and from the protection and easy passage which in the course of time they afford to rats, mice, weasels, &c. whose activity in the night-time affords the foundation of numerous apprehensions and fancies of the credulous.

4. To spontaneous combustions or detonations, which produce occasional lights and noises, or, under unchanged circumstances, recurring lights and noises, chiefly claiming attention in the night. Thus houses shut up and unaired are apt, from the putrefaction of animal and vegetable matter, to generate hydrogen gas, the accidental combustion of which by contact with phosphoric matter, naturally generated in the same situation, will produce those effects of lights and noises heard in  empty houses. So Church-yards, Churches in which the dead are buried, Cemeteries, and Ruins of old buildings, must frequently give out large quantities of these gases; and consequently, from exactly similar causes, they are likely to produce the very effects which we witness in the will-o’-the-wisp, or in hydrogen gas when inflamed during calm weather in marshy situations.

5. To the prevailing belief that effects, which cannot readily be accounted for, or which are caused by the contact of the invisible fluids or media always in action in the great laboratory of nature, are produced by the agency of spirits or demons; which belief, concurring with the unknown causes of the effects, and affording a ready solution of difficulties, prevents further inquiry, silences reasoning, and tends in consequence to sustain the prevailing errors and superstitions.

Such are the general causes of ghosts, spirits, charms, miracles, and supernatural appearances. They all arise either from hallucinations of the mind or senses;  from the mutual action of the natural, though invisible, powers of gaseous and ethereal fluids; from the delusions of ignorance, implicit faith, or the absence of all reasoning.

Sir Richard Phillips, A Morning’s Walk From London To Kew (1817)