Smiling can imply a sense of humour and a state of amusement, as in this painting of Falstaff by Eduard von Grützner.
Humour or humor (see spelling differences) is the tendency of particular cognitive experiences to provoke laughter and provide amusement. The term derives from the humoral medicine of the ancient Greeks, which taught that the balance of fluids in the human body, known as humours (Latin: umor, "body fluid"), control human health and emotion.
People of all ages and cultures respond to humour. The majority of people are able to experience humour, i.e., to be amused, to laugh or smile at something funny, and thus they are considered to have a sense of humour. The hypothetical person lacking a sense of humour would likely find the behaviour induced by humour to be inexplicable, strange, or even irrational. Though ultimately decided by personal taste, the extent to which an individual will find something humorous depends upon a host of variables, including geographical location, culture, maturity, level of education, intelligence and context. For example, young children may favour slapstick, such as Punch and Judy puppet shows or cartoons such as Tom and Jerry. Satire may rely more on understanding the target of the humour and thus tends to appeal to more mature audiences. Nonsatirical humour can be specifically termed "recreational drollery".
Many theories exist about what humour is and what social function it serves. The prevailing types of theories attempting to account for the existence of humour include psychological theories, the vast majority of which consider humour-induced behaviour to be very healthy; spiritual theories, which may, for instance, consider humour to be a "gift from God"; and theories which consider humour to be an unexplainable mystery, very much like a mystical experience.
Arthur Schopenhauer lamented the misuse of the term "humour" (a German loanword from English) to mean any type of comedy. However, both "humour" and "comic" are often used when theorising about the subject. The connotations of "humour" as opposed to "comic" are said to be that of response versus stimulus. Additionally, "humour" was thought to include a combination of ridiculousness and wit in an individual; the paradigmatic case being Shakespeare's Sir John Falstaff. The French were slow to adopt the term "humour"; in French, "humeur" and "humour" are still two different words, the former referring to a person's mood or to the archaic concept of the four humours.
Western humour theory begins with Plato, who attributed to Socrates (as a semihistorical dialogue character) in the Philebus (p. 49b) the view that the essence of the ridiculous is an ignorance in the weak, who are thus unable to retaliate when ridiculed. Later, in Greek philosophy, Aristotle, in the Poetics (1449a, pp. 34–35), suggested that an ugliness that does not disgust is fundamental to humour.
In ancient Sanskrit drama, Bharata Muni's Natya Shastra defined humour (hāsyam) as one of the nine nava rasas, or principle rasas (emotional responses), which can be inspired in the audience by bhavas, the imitations of emotions that the actors perform. Each rasa was associated with a specific bhavas portrayed on stage. In the case of humour, it was associated with mirth (hasya).
The terms "comedy" and "satire" became synonymous after Aristotle's Poetics was translated into Arabic in the medieval Islamic world, where it was elaborated upon by Arabic writers and Islamic philosophers such as Abu Bischr, his pupil Al-Farabi, Avicenna, and Averroes. Due to cultural differences, they disassociated comedy from Greek dramatic representation, and instead identified it with Arabic poetic themes and forms, such as hija (satirical poetry). They viewed comedy as simply the "art of reprehension" and made no reference to light and cheerful events or troublous beginnings and happy endings associated with classical Greek comedy. After the Latin translations of the 12th century, the term "comedy" thus gained a new semantic meaning in Medieval literature.
The Incongruity Theory originated mostly with Kant, who claimed that the comic is an expectation that comes to nothing. Henri Bergson attempted to perfect incongruity by reducing it to the "living" and "mechanical".
An incongruity like Bergson's, in things juxtaposed simultaneously, is still in vogue. This is often debated against theories of the shifts in perspectives in humour; hence, the debate in the series Humor Research between John Morreall and Robert Latta. Morreall presented mostly simultaneous juxtapositions, with Latta countering that it requires a "cognitive shift" created by a discovery or solution to a puzzle or problem. Latta is criticised for having reduced jokes' essence to their own puzzling aspect.
Humour frequently contains an unexpected, often sudden, shift in perspective, which gets assimilated by the Incongruity Theory. This view has been defended by Latta (1998) and by Brian Boyd (2004). Boyd views the shift as from seriousness to play. Nearly anything can be the object of this perspective twist; it is, however, in the areas of human creativity (science and art being the varieties) that the shift results from "structure mapping" (termed "bisociation" by Koestler) to create novel meanings. Arthur Koestler argues that humour results when two different frames of reference are set up and a collision is engineered between them.
Tony Veale, who takes a more formalised computational approach than Koestler, has written on the role of metaphor and metonymy in humour, using inspiration from Koestler as well as from Dedre Gentner's theory of structure-mapping, George Lakoff and Mark Johnson's theory of conceptual metaphor, and Mark Turner and Gilles Fauconnier's theory of conceptual blending.
Some claim that humour cannot or should not be explained. Author E.B. White once said, "Humor can be dissected as a frog can, but the thing dies in the process and the innards are discouraging to any but the pure scientific mind."
As with any form of art, acceptance depends on social demographics and varies from person to person. Throughout history, comedy has been used as a form of entertainment all over the world, whether in the courts of the Western kings or the villages of the Far East. Both a social etiquette and a certain intelligence can be displayed through forms of wit and sarcasm. Eighteenth-century German author Georg Lichtenberg said that "the more you know humour, the more you become demanding in fineness."  Evolutionary explanation of humour
Alastair Clarke explains: "The theory is an evolutionary and cognitive explanation of how and why any individual finds anything funny. Effectively, it explains that humour occurs when the brain recognises a pattern that surprises it, and that recognition of this sort is rewarded with the experience of the humorous response, an element of which is broadcast as laughter." The theory further identifies the importance of pattern recognition in human evolution: "An ability to recognise patterns instantly and unconsciously has proved a fundamental weapon in the cognitive arsenal of human beings. The humorous reward has encouraged the development of such faculties, leading to the unique perceptual and intellectual abilities of our species."