DELUSIONS are synonymous, in a popular way, with false beliefs. Thus, we often speak of eccentric opinions, of fanatical or extravagant creeds, as delusions. In a certain sense probably all mankind cherish innumerable delusions. In a strictly medical and medico-legal sense, however, the term is applied only to false beliefs in respect to clearly-established, indisputable facts. Thus, a man who believes in Spiritualism or even in metempsychosis, or in the divinity of a certain personage, is not medically deluded; whereas, one who believes that a bare court is a flowering garden or that he himself is divine is deluded. The essential element in the conception of delusion is belief or conviction on the patient's part; and that is why delusions mean that the psychic functions are deeply and seriously impaired. Delusions may be conveniently divided into ideal and sensorial. {23} (_a_) Ideal delusions are false ideas or concepts arising more or less spontaneously, or by morbid association in the subject's mind. For example: he believes that he is a god, that he has millions of money, that his soul is lost, that he has a thousand children, etc. Many of the delirious ideas experienced by insane patients are delusions, and so to a certain extent (subject to temporary corrections by reasoning and demonstration) are the notions of hypochondriacs about their health. (_b_) Sensorial delusions are such as are founded upon illusions and hallucinations. The moment a subject is convinced of the reality of an illusion or hallucination, believes in its actuality, he is said to have a delusion. The change from illusion and hallucination to the state of sensorial delusion indicates a deeper psychic alteration—a failure of critical capacity or judgment. Examples: A man imagines the stump of a tree in front of him to be a human being, but by reasoning, by closer visual inspection, or by palpation he concludes that it is a tree, after all; this is a simple illusion. If he persists, in spite of argument and demonstration, in his assertion that the stump is a human being, he is said to have a delusion or to be deluded. If a person sees wholly imaginary flowers or hears imaginary voices, as long as he is capable of recognizing the falsity or want of actuality of these images or sounds he has a simple hallucination; if he ceases to make the necessary correction, and believes the flowers and voices to really exist, he has sensorial delusions. It should be borne in mind that sane persons may have hallucinations, and that some insane have no sensorial delusions; also, that some insane are capable of correcting, for a time at least or when closely questioned, their illusions and hallucinations. Apart from these exceptional conditions, delusions, sensorial and ideal, are most important symptoms of insanity. We also meet temporary delusions in toxic conditions (from Indian hemp, alcohol, etc.) and in the delirium of acute general