Records of abnormal mental phenomena reach back to the very dawn of history, and are to be found in the oldest books in both the Eastern and the Western world. Thus, in the Old Testament we read of Saul's recurring periods of depression, when ‘the evil spirit from the Lord’ was upon him. We read, again, of the delirium of Nebuchadnezzar, in which he believed himself changed into an animal—he ‘did eat grass like an ox, and his body was wet with the dew of heaven, till his hairs grew like the feathers of eagles, and his nails like birds’ claws'. To turn to the West, abnormal mental phenomena frequently appear in the pages of Homer. Ajax was tortured by the Furies till he fell upon his own sword, and we are told that Ulysses simulated madness in order to justify his abstention from the Trojan war. The famous oracles are not altogether attributable to fraud, but are probably partly to be explained as hysterical manifestations similar to those found in the ‘Medium’ of the present day.

Side by side with the abnormal phenomenon we invariably find its attempted explanation, because the demand for explanation is a fundamental character of the human mind. Throughout the history of mental disorder, and particularly with regard to the more severe forms of mental disorder comprised under the term ‘insanity’, this demand constantly makes itself felt, endeavouring to obtain satisfaction by the construction of explanations in harmony with the general thought and knowledge of the period.