What Is an Illusion?
Under certain circumstance, normal people can experience an illusion. It consists of an object/stimulus that is falsely perceived either because of the lack of clarity of the object/stimulus or due to the psychological state of the person. Thus an innocuous silhouette or sound may be perceived as something threatening be someone in a suggestible condition. When the psychological state is morbid, then what is perceived may be misinterpreted e.g. in idea of reference. The misperception is understandable and secondary to an underlying morbid state.