To the world, the alpha and omega of this piece of philosophy were furnished by the publication of the case of Nicolai, the bookseller of Berlin. Its details were read before the Academy of Sciences at Berlin, in 1799. The _substance_ ran thus. Nicolai had had some family troubles which much annoyed him. Then, on the 21st of February 1791, there stood before him, at the distance of ten paces, the ghost of his eldest son. He pointed at it, directing his wife to look. She saw it not, and tried to convince him that it was an illusion. In a quarter of an hour it vanished. In the afternoon, at four o'clock, it came again. Nicolai was alone. He went to his wife's room--the ghost followed him. About six other apparitions joined the first, and they walked about, among, and through each other. After some days the apparition of his son stayed away; but its place was filled with the figures of a number of persons, some known, some unknown to Nicolai--some of dead, others of living persons. The known ones were distant acquaintances only. The figures of none of Nicolai's habitual friends were there. The appearances were almost always human: exceptionally, a man on horseback, with dogs and birds would present themselves. The apparitions came mostly after dinner, at the commencement of digestion. They were just like real persons; the colouring a thought fainter. The apparitions were equally distinct whether Nicolai was alone or in society, by day as in the dark, in his own house or those of others; but in the latter case they were less frequent, and they very seldom presented themselves in the streets. During the first eight days they seemed to take very little notice of each other, but walked about like people at a fair, only here and there communing with each other. They took no notice of Nicolai, or of his remarks about them to his wife and physician. No effort of his would dismiss them, or bring an absent one back. When he shut his eyes, they sometimes disappeared, sometimes remained; when he opened his eyes, they were there as before. After a week they became more numerous, and began to converse. They conversed with each other, and then addressed him. Their remarks were short and unconnected, but sensible and civil. His acquaintances inquired after his health, and expressed sympathy for him, and spoke in terms comforting him. The apparitions were most conversible when he was alone; nevertheless they mingled in the conversation when others were by, and their voices had the same sound as those of real persons. This illusion went on thus from the 24th of February to the 20th of April; so that Nicolai, who was in good bodily health, had time to become tranquillised about them, and to observe them at his ease. At last they rather amused him. Then the doctors thought of an efficient plan of treatment. They prescribed leeches: and then followed the _denouement_ to this interesting representation. The apparitions became pale and vanished.