*Classes of Sensations.*—Perhaps as many as twenty distinct sensations, such as pain, hunger, touch, etc., are recognized. If these are studied with reference to their origin, it will be seen that some of them result from the action of definite forms of stimuli upon the neurons terminating in sense organs; while the others, as a rule, arise from the action of indefinite stimuli upon neurons in parts of the body that do not possess sense organs. The members of the first class—and these include the sensations of touch, temperature, taste, smell, hearing, and sight—are known as the _special_ sensations. The others, including the sensations of pain, hunger, thirst, nausea, fatigue, comfort, discomfort, and those of disease, are known as _organic_, or general, sensations. These two classes of sensations differ in their purpose in the body as well as in the manner of their origin. *Purposes of Sensations.*—Any given sensation is related to the stimulus which excites it as an _effect_ to a _cause_. It starts up or stops, increases in intensity or diminishes, according to the action of the exciting stimulus. As the stimuli are outside of the nervous system, and in the majority of cases outside of the body, the sensations indicate to the mind what is taking place either in the body itself or in its surroundings. They supply, in other words, the means through which the mind acquires information. By means of the special sensations, a knowledge of the physical surroundings of the body is gained, and through the organic sensations the needs of the body and the state of the various organs are indicated. In general, sensations are made to serve two great purposes in the body, as follows: 1. They provide the necessary conditions for intelligent and purposeful action on the part of the body. 2. They supply the basis for the higher mental activities, as perception, memory, thought, imagination, and emotion. Intelligent action is impossible without a knowledge both of the bodily organs and of the body’s surroundings. Protection and the regulation of the work of an organ necessitate a knowledge of its condition, while the adapting and adjusting of the body to its surroundings require a knowledge of what those surroundings are. The dependence of all the higher forms of mental activity upon sensations is recognized by psychologists and is easily demonstrated by a study of the manner in which we acquire knowledge. "Without sensation there can be no thought."