 The Relief Theory is an hydraulic explanation in which laughter does in the nervous system what a pressure-relief valve does in a steam boiler. The theory was sketched in Lord Shaftesbury’s 1709 essay “An Essay on the Freedom of Wit and Humor,” the first publication in which humor is used in its modern sense of funniness. Scientists at the time knew that nerves connect the brain with the sense organs and muscles, but they thought that nerves carried “animal spirits”—gases and liquids such as air and blood. John Locke (1690, Book 3, ch. 9, para.16), for instance, describes animal spirits as “fluid and subtile Matter, passing through the Conduits of the Nerves.” 