The general opinion held at that time was that the phenomena of nightmare were due to excess of bile and dryness of the blood. This view originated with Hippocrates, but was more or less modified by subsequent writers. After the establishment of Christianity, the conviction began to prevail that during an attack of nightmare the subject was visited by a demon, who, for the time being, took possession of his body. Oribasius, in the fourth century, combated this idea, and endeavored to show that it was a severe disease, which, if not cured, might lead to apoplexy, mania, or epilepsy. He located it in the head. Aetius also denied the existence of demoniacal agency in nightmare. He considered it as a prelude to epilepsy, mania, or paralysis. During the middle ages nightmare was attributed to the power of the devil. Imps, male and female, called incubi and succubi respectively, were supposed to be the active agents in producing the affection. The treatment was in accordance with the theory, and consisted of prayers and exorcisms. Not unfrequently the subject of the disease perished at the stake for the alleged crime of having sexual intercourse with incubi or succubi, according to sex. Even in later times many persons have been found who believed implicitly in the reality of the visions which they experienced during an attack of nightmare. Thus Jansen[104] relates that a clergyman came to consult him. “Monsieur,” said he, “if you do not help me I shall certainly go into a decline, as you see I am thin and pale,--in fact, I am only skin and bone; naturally I am robust, and of good appearance; now I am scarcely more than the shadow of a man.” “What is the matter with you?” said Jansen. “And to what do you attribute your disease?” “I will tell you,” answered the clergyman, “and you will assuredly be astonished at my story. Almost every night a woman, whose figure is not unknown to me, comes and throws herself on my breast, and embraces me with such power that I can scarcely breathe. I endeavor to cry out, but she stifles my voice, and the more I try the less successful I am. I can neither use my arms to defend myself, nor my legs to escape. She holds me bound and immovable.” “But,” said the doctor, “what you relate is not in the least surprising. Your visitor is an imaginary being, a shade, a phantom, an effect of your imagination.”