The search for a sixth sense: the cases for vestibular, muscle, and temperature senses

The five senses of sight, hearing, smell, taste, and touch, enumerated by Aristotle, were incremented in the early-nineteenth century by the muscle sense, multiple dimensions of touch, and a movement sense. Aristotle explicitly excluded a sixth sense, and five remains the number of senses in popular imagination. The division of touch into several sensations was entertained and rejected by Aristotle, but it was given anatomical, physiological and psychophysical support in the late-nineteenth century. A separate muscle sense was proposed in the late-eighteenth century, with experimental evidence to support it. However, before these developments, behavioral evidence of the vestibular (movement) sense was available from studies of vertigo, although it was not integrated with the anatomy and physiology of the labyrinth until the nineteenth century. The history of the search for a sixth sense is outlined, and the evidence adduced to support the divisions is assessed. Behavioral evidence generally has been accorded less weight than that from anatomy and physiology.