James Sully, answered from the texts and cited to the page.
Laughter is no trivial thing for the psychologist, but a natural phenomenon with its own causes, forms, and history, rooted deep in our animal and social nature.1 I trace it to the play-attitude — that mood of pleasant relaxation and freedom in which we are not in earnest — and to the perception of a certain agreeable incongruity, a sudden gay recognition of the unfitting, the out-of-place, the comic contradiction, enjoyed in the security of play.
The child laughs first in the romp and the tickle, in mock-fear and the game of surprise; the grown mind laughs at the incongruities of wit and the ludicrous, but the root is one: a pleasurable release of energy in the presence of the harmlessly discordant. Laughter has its value, too — it lightens the strain of life, corrects by ridicule, and binds people in shared amusement.
To understand it we must watch it grow in the child and observe its play in society, and explain it, like all else in mind, by natural processes and not by mystery.234
We may further illustrate and verify this generalisation respecting the causes of joyous laughter by an examination of some of the more familiar circumstances in which this is wont to occur. Here we shall of course be dealing with the early and unsophisticated mind. Properly drilled "grown-ups" but rarely exhibit the phenomenon in its full intensity.An Essay on Laughter
Much care is needed in the interpretation of such expressive reactions. A small boy of eighteen months laughed when his pants slipped down. But this may only have resulted from a sense of the fun of the irregularity of the proceeding, aided perhaps by others' amusement.An Essay on Laughter
It is fairly certain that we have to do in this case with a double or "divided" consciousness. And, as has been illustrated above, laughter is wont to hover about the domain of the serious.An Essay on Laughter
Is this understandable unless we suppose that laughter at a person is instinctively interpreted as an assertion of superiority over him? It would seem then to be a reasonable view, that if laughter in ordinary cases involves superiority, and is so regarded by its object, the enjoyment of it by its subject will be very apt to bring with it a taste of superiority.An Essay on Laughter
Sully's Essay on Laughter is a substantial naturalistic study of the comic, combining the incongruity and relief theories with his developmental and observational approach (laughter traced from the child's play), and it remains a respected contribution to the psychology of humor. It appeared amid a rich moment for the subject: Bergson's Le Rire (1900), which explained the comic as the correction of the mechanical encrusted on the living, and Freud's Jokes and Their Relation to the Unconscious (1905), which read jokes as releases of repressed energy, were its great contemporaries. The modern psychology of humor still works within the incongruity, relief, and superiority families of theory that Sully synthesized; his play-and-incongruity account is dated in idiom but not superseded in the way his associationism is.