Vital heat, also called innate or natural heat, or calidum innatum, is a term in Ancient Greek medicine and philosophy that has generally referred to the heat produced within the body, usually the heat produced by the heart and the circulatory system. Vital heat was a somewhat controversial subject because it was formerly believed that heat was acquired by an outside source such as the element of fire. Origin of concept It was a previously accepted concept that heat was absorbed through external sources; however, the concept of vital heat was more or less stumbled upon by a physiological observation that associates cold with the dead and heat with the living. "For the concept of vital heat we may – somewhat arbitrarily – take our starting point in Parmenides. His correlation of dead with cold, alive with warm, may not have been primarily intended as a contribution to physiology, yet the physiological significance of this thought was perceived by his successors; witness Empedocles, who taught 'sleep comes about when the heat of the blood is cooled to the proper degree, death when it becomes altogether cold'". Aristotle would eventually modify this doctrine stating that "sleep is a temporary overpowering of the inner heat by other factors in the body, death its final extinction." Vital heat and ancient medicine According to Ancient Greek physicians, vital heat was produced by the heart, maintained by the pneuma (air, breath, spirit or soul), and circulated throughout the body by blood vessels, which were thought to be intact tubes using blood to transmit heat. Aristotle supported this argument by showing that when the heart is made cold compared to other organs, the individual dies. He believed that the heat produced in the heart caused blood to react in a similar way to boiling, expanding out through the blood vessels with every beat. This extreme heat, according to him, can lead to a self-consuming flame if it is not cooled by air from the lungs. Galen wrote in On the Usefulness of the Parts of the Body (170): "The heart is, as it were, the hearthstone and source of the innate heat by which the animal is governed." In the 11th century, Avicenna agreed with this notion, stating that the heart produced breath, the "vital power or innate heat" within the body, in his Canon of Medicine. Later, the term innate heat was attributed to friction