In response to a stimulus, a sensation is tridimensional: qualitative, quantitative, and affective. The affective part of sensation, pleasure or displeasure, depends on the qualities of the stimulus. Within a narrow range of intensity, chemical, thermal, and mechanical stimuli are able to arouse pleasure. In addition, pleasure depends on the internal state of the subject. This is easily observed in the case of temperature: pleasure is aroused by a warm stimulus in a hypothermic subject and by a cold stimulus in a hyperthermic subject. This property of a given stimulus to arouse pleasure or displeasure according to the internal state of the subject is termed alliethesia. Alliesthesia is also produced by chemical and mechanical stimuli. Acquired preferences or aversions for alimentary stimuli represent a case of alliesthesia. In the same way, the capacity of any indifferent stimulus to become rewarding, or punishing, by association with some reward or punishment, is also a case of alliethesia. In all cases, pleasure is a sign of a stimulus useful to the subject; displeasure a sign of danger. Usefulness and danger are judged by the central nervous system with reference to homeostasis and the set point of the implied regulation. Pleasure and displeasure thus appear to motivate useful behaviors.