In the normal brain certain impressions are received from the special senses: impressions of sight or of hearing, for example. These impressions are called conscious perceptions, and the healthy brain groups them together and forms concepts. For instance, you see something which is flat and shiny with square-cut edges. You touch it, and learn that it is cold, smooth, and hard. Lift it and you find it heavy. Grouping together your sense perceptions you form the concept, and decide that the object is a piece of marble. Again, you enter a dimly lighted room and see a figure in a corner the height of a woman, with a gown like a woman's. You approach it, speak to it and get no reply, and you find you can walk directly through it, for it is a shadow. Perhaps you were frightened. Perhaps you imagined she was a thief. Your first judgment was wrong and you correct it. The insane person, however, has defective mental processes. He cannot group together his perceptions and form proper conceptions. His imagination runs riot. His emotions of fear or anger are not easily limited. He has to some extent lost the control over his mental actions that you and other people possess if your brains are normal. The insane man will insist that there is a woman there, and not a shadow, and to his mind it is not absurd to walk directly through this person. He cannot correct the wrong idea. Such a wrongly interpreted sense perception is called an illusion. Another example of illusion is the mistaking the whistle of a locomotive for the shriek of a pursuing assassin. _What Hallucinations Are_ The insane man may also suffer from hallucinations. A hallucination is a false perception arising without external sensory experience. In a hallucination of sight, the disease in the brain causes irritation to be carried to the sight-centers of the brain, with a result that is similar to the impression carried to the same centers by the optic nerves when light is reflected into the eyes from some object. An insane man may be deluded with the belief that he sees a face against the wall where there is nothing at all. When the air is pure and sweet and no odor is discoverable, he may smell feathers burning, and thus reveal his hallucination of smell. _Delusions Common to Insanity_