Henry Ansgar Kelly (pp. 139-140 of Chaucerian Tragedy ) makes this important historical comment at the end of his analysis of Chaucer’s Troilus and Criseyde :
“The selection introduction of Aristotelian criteria of excellent in tragedy has been a source of untold confusion in modern discussions of tragedy. For Aristotle, the ‘raw material’ to be classified, that is, the mass of plays or stories already categorized as tragedies, was distinguished by ‘seriousness,’ and included a large number of cases with happy endings. In the Middle Ages, according to several authorities, the unhappy ending was one of the main generic distinctions of tragedy. This idea has become so ingrained in us that we tend to assume that it was also Aristotle’s idea: that is, we assume that Aristotle was working from a medieval definition of tragedy, and we tend to edit out or ignore his recognition of tragedies with happy endings: especially, as already noted, his discussion in the fourteenth chapter of the Poetics where he says that a tragedy like Euripides’s Iphigeneia in Tauris best achieves the desired tragic effects.
“It is Chaucer rather than Aristotle who sets forth the acceptable limits of our modern idea of tragedy, and it is Chaucer who can be said to have fixed these limits for the modern world. For Chaucer was not simply handing on standard medieval doctrine. There was no standard medieval doctrine on the point; or, if there was, it was not what Chaucer came up with. Ideas of tragedy were comparatively rare in the Middle Ages . . . , and they came with great variety. Some authors, like Dante, considered tragedy mainly a matter of style, poetic form, and static content, having nothing to do with disastrous events. It is true, however, that many, or most, definitions of tragedy included an unhappy ending; but the ending usually referred to deserved retribution visited upon the wicked. It was Chaucer’s good fortune that he received from his Boethius glossator an unrestricted defintion of tragedy, which left every sort of misfortune eligible for inclusion. Chaucer’s definition corresponds to the modern everyday idea of tragedy, the range of which can be tested by considering the applications of the expression ‘What a tragedy!’”