In this book of 1825, Samuel Hibbert (1782–1848) attempts to uncover the physical or physiological causes which might account for claims of seeing ghosts and other apparitions. Hibbert trained as a doctor, and uses anecdotal and case-study evidence to show that external physical circumstances - such as the use of stimulants, brain inflammation, hallucination during fever, or alcohol withdrawal - are most likely to be the causes of apparent sightings of supernatural phenomena. He explores the power of suggestion, whether derived from superstitions, folk tales or biblical imagery, on the imagination of the impressionable. Using the idea that the train of thought can be stimulated or depressed, and that different levels of semi-consciousness can admit of different levels of contemplation and concentration, Hibbert hypothesises that for each apparition or ghostly spectre there is a rational explanation.