The ancient theory of "animal spirits" (pneuma psychikon in Greek; spiritus animalisin Latin) was first proposed by Alexandrian physicians in the third century BCE. Animal spirits were thought to be weightless, invisible entities that flowed through the hollow nerves to mediate the functioning of the body. The animal spirits theory was related to the notion of the four humours (blood, phlegm, and yellow and black bile), and was popularised by the Roman physician Galen (c. 129 -216) in the second century AD. Because of Galen, animal spirits dominated thinking about the nervous system for 1,500 years; they were exorcised very recently - it was only during the latter part of the 18th century that investigators began to decipher the electrochemical language of the nervous system.

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