As definitions go, this is pretty thin. Although Aristotle makes it clear that laughter is a form of disdain, distinct from mockery, he does not explain what sorts of errors and disgrace make a person ridiculous, what sort of pleasure laughter confers, whether any other dramatic devices are needed to elicit a chuckle, or if any other types of laughter are possible. He probably intended to answer these in another part of his treatise. At various points, he promises that there is more to come; some later authorities (Diogenes Laertius, Boethius, etc.) indicate that, in its original form, the Poetics contained at least two books. But at some point in late antiquity the second was lost.
For more than 1,000 years scarcely a trace of the lost treatise remained in the Latin West, beyond a handful of second-hand remarks. In 1839, however, a manuscript was found in the Bibliothèque nationale de France which looked like a summary of the missing text. Since then, this has been used as the basis for a possible reconstruction. But the manuscript is so garbled that some scholars doubt its authenticity and feel that Aristotle’s full philosophy of laughter may be gone forever