The grammarian, Diomedes, has preserved the definition of tragedy formulated by Theophrastus, Aristotle's successor as head of the Peripatetic school. According to this definition, tragedy is "a change in the fortune of a hero."[119] A Greek definition of comedy preserved by Diomedes, and ascribed to Theophrastus also,[120] speaks of comedy as dealing with private and civil fortunes, without the element of danger. This seems to have been the accepted Roman notion of comedy. In the treatise of Euanthius-Donatus, comedy is said to deal with the common fortunes of men, to begin turbulently, but to end tranquilly and happily; tragedy, on the other hand, has only mighty personages, and ends terribly; its subject is often historical, while that of comedy is always invented by the poet.[121] The third book of Diomedes's _Ars Grammatica_, based on Suetonius's tractate _De Poetis_ (written in the second century A.D.), distinguishes tragedy from comedy in that only heroes, great leaders, and kings are introduced in tragedy, while in comedy the characters are humble and private persons; in the former, lamentations, exiles, bloodshed predominate, in the latter, love affairs and seductions.[122] Isidore of Seville, in the seventh century, says very much the same thing: "Comic poets treat of the acts of private men, while tragic poets treat of public matters and the histories of kings; tragic themes are based on sorrowful affairs, comic themes on joyful ones."[123] In another place he speaks of tragedy as dealing with the ancient deeds and misdeeds of infamous kings, and of comedy as dealing with the actions of private men, and with the defilement of maidens and the love affairs of strumpets.[124] In the _Catholicon_ of Johannes Januensis de Balbis (1286) tragedy and comedy are distinguished on similar grounds: tragedy deals only with kings and princes, comedy with private citizens; the style of the former is elevated, that of the latter humble; comedy begins sorrowfully and ends joyfully, tragedy begins joyfully and ends miserably and terribly.[125] For Dante, any poem written in an elevated and sublime style, beginning happily and ending in misery and terror, is a tragedy; his own great vision, written as it is in the vernacular, and beginning in hell and ending gloriously in paradise, he calls a comedy.[126] It appears, therefore, that during the post-classic period and throughout the Middle Ages, comedy and tragedy were distinguished on any or all of the following grounds:-- i. The characters in tragedy are kings, princes, or great leaders; those in comedy, humble persons and private citizens. ii. Tragedy deals with great and terrible actions; comedy with familiar and domestic actions. iii. Tragedy begins happily and ends terribly; comedy begins rather turbulently and ends joyfully. iv. The style and diction of tragedy are elevated and sublime; while those of comedy are humble and colloquial.