The object of this paper is to show the important part played by introspection in the mechanism of certain delusions, and especially in the genesis of psycho-motor hallucinations. The complete notes of a case carefully observed in this connection are given—that of a woman aged forty-three years. Tormented by the thought of wrong-doing, by ideas of doubt and fear, the patient was especially anxious to analyse and explain her mental condition. She at first is satisfied that her ideas take birth within her mind, then believes that she is selfhypnotised; a stage further she suspects the domination of some indefinite power which directs her thoughts, experiments upon her, hypnotises her, and speaks within her. If she thinks evil of certain persons she hears them “inside “herself replying, insulting her. At times she moves her lips when speaking her own ideas, and is conscious of a conversation, an asking and a replying, going on in her head. Ideas of guilt are generally associated with these phenomena, and she asks forgiveness for these ideas, which spring up without the intervention of her will. The explanation which satisfies her best is that she is hypnotised and made to think, and the measure of her guilt is the consent which she gives to certain of these ideas—this consent being withheld in the case of others. The thesis which the authors endeavour to prove is that the interior language is the principal source of a considerable number, if not of most, psycho-motor hallucinations. They urge the importance of a careful study of the mental life of patients, and the fallacy of restricting observations to somatic examination alone.