Unless suffering from one of those rare forms of hereditary indifference to pain, no human is without the experience of pain. Yet humans have always had difficulty in conveying a unified concept of pain since it can include subjective states ranging from mere unpleasantness to extreme physical agony, or to the feeling of sadness and desolation accompanying an episode of major depression. Plato and Aristotle did not regard pain as an elemental sensation like touch or vision but rather saw pain and pleasure as contrasting elements vying with one another for the maintenance of internal wellbeing of the individual by operating on the soul, which was thought to be located in the liver or heart. For Aristotle, pain arose from ripples in the heart and blood vessels, not from the activity of the reasoning brain. Perhaps we can still see crude echoes of the Aristotelian position in modern suggestions that pain is no more than a disruption of bodily homeostasis, akin to that associated with dysautonomia and other visceral disturbances.

It is to Galen, writing more than 450 years after Aristotle, that we owe the recognition that sensory impressions, including those leading to pain, are carried by nerves to the brain and Galen described carrying out cordotomies in animals in order to demonstrate the key role of the spinal cord in the conduction of painful impressions to the brain.