Theories of humour are generally classified under three approaches
Perhaps the majority of classical philosophers who considered humour at any depth had the cutting humour of satire and the like significantly in mind, so it was perhaps natural to suspect (intellectuals are good at suspecting) that a sense of superiority was at the heart of the phenomenon.
Hobbes is frequently cited as the typical example of this approach:
The passion of laughter is nothing else but sudden glory arising from some sudden conception of some eminency in ourselves, by comparison with the infirmity of others, or with our own formerly.
That humour can have at its heart such a sense is evident. However, often humour seems to work in reverse as we laugh at ourselves or with a character, for example when a downtrodden or bungling character surprisingly succeeds.
(When, in part 3, I present my “criteria for discerning the presence of humour” this approach will be reflected in the suggestion that a marked sense of inferiority and superiority can signal humour as well as sometimes in surprise, incongruity or a focus on human pretentions.)
Currently the commonest approach sees humour as located in a sense of something or someone out of place, or unfitting. This incongruity associated with surprise produces laughter.
An early example of this approach is found in Aristotle and the theory was given classic expression by Kant:
In everything that is to excite a lively convulsive laugh there must be something absurd (in which the understanding, therefore, can find no satisfaction). Laughter is an affection arising from the sudden transformation of a strained expectation into nothing. This transformation, which is certainly not enjoyable to the understanding, yet indirectly gives it very active enjoyment for a moment. Therefore its cause must consist in the influence of the representation upon the body, and the reflex effect of this upon the mind.
Although this linkage of humour with incongruity seems to fit a wider selection of what today we would classify as humorous, again the theory is less than entirely satisfying. It does not well describe precisely those examples that the superiority theory best accounts for.
It will be reflected in the criteria of ingenuity, surprise, incongruity and perhaps disguise.
Although theories that link humour with the relief of tensions or energy that have previously been accumulated may help to understand the physiological phenomenon of laughter, and despite beoing promulgated by such luminaries as The 3rd Earl of Shaftesbury, Herbert Spenser, John Dewey and Sigmund Freud relief theories are generally depreciated today as descriptions of humour (as opposed more narrowly the mechanisms of laughter), and this attitude is reflected in the absense of influence from these theories on the criteria I will be proposing.