Alliesthesia refers to changes in a person's milieu interieur that determine whether a stimulus is judges as pleasant or unpleasant. It is stimuli and behaviors that restore homeostasis feel pleasant while those that disrupt homeostasis feel unpleasant. An example of this would be drink and eating. Drinking and eating are pleasureable when thierst and hungry because those activities restore homeostatis. An example would be a person's need for water. Depending on how much water they need at a certain time, that depends on the pleasure level that water will have. For example if a person was extremely thirsty, they would desrcibe drinking water as "delicious" or "exquisite". Alliesthesia motivated the person to drink to restore their body's water level.
Alliesthesia always affects something called thermoregulation. Thermoregilation deals with regulating the body's temperature. The book talks about and example that explains the role of alliesthesia. It involves participants submerging their hands in either cold water or warm water. Participants put their hands in a range of either cold or warm water and rated the baths on an unpleasantess-pleasantness scale. Ratings of the water baths dependeed on the participants core body temperature. The participants who were cold judged the cooler baths as unpleasant and the warmer baths as pleasant. The participants who were warm judged the cooler baths as pleasant and the warmer baths as unpleasant. Participants who had normal core body temperature judged a water bath as unpleasant only when it deviated greatly from normal skin temperature. Baths that restore body temperature to homeostasis are those that feel pleasant. Baths are felt to be unpleasant if they cause actual body temperature to deviate further from set point temperature.