Unlike hypochondriasis, in which there is no physical problem, conversion disorders involve an actual physical disturbance, such as the inability to see or hear or to move an arm or leg. The cause of such a physical disturbance is purely psychological; there is no biological reason for the problem. Some of Freud’s classic cases involved conversion disorders. For instance, one of Freud’s patients suddenly became unable to use her arm, without any apparent physiological cause. Later, just as abruptly, the problem disappeared.