The first reference in modern literature to the doctrine of the unity of time is to be found in Giraldi Cintio's _Discorso sulle Comedie e sulle Tragedie_. He says that comedy and tragedy agree, among other things, in the limitation of the action to one day or but little more;[179] and he has thus for the first time converted Aristotle's statement of an historical fact into a dramatic law. Moreover, he has changed Aristotle's phrase, that tragedy limits itself "to a single revolution of the sun," into the more definite expression of "a single day." He points out that Euripides, in the _Heraclidae_, on account of the long distance between the places in the action, had been unable to limit the action to one day. Now, as Aristotle must have known many of the best Greek dramas which are now lost, it was probably in keeping with the practice of such dramas that their actions were not strictly confined within the limits of one day. Aristotle, therefore, intentionally allowed the drama a slightly longer space of time than a single day. The unity of time, accordingly, becomes a part of the theory of the drama between 1540 and 1545, but it was not until almost exactly a century later that it became an invariable rule of the dramatic literature of France and of the world. In Robortelli (1548) we find Aristotle's phrase, "a single revolution of the sun," restricted to the artificial day of twelve hours; for as tragedy can contain only one single and continuous action, and as people are accustomed to sleep in the night, it follows that the tragic action cannot be continued beyond one artificial day. This holds good of comedy as well as tragedy, for the length of the fable in each is the same.[180] Segni (1549) differs from Robortelli, however, in regarding a single revolution of the sun as referring not to the artificial day of twelve hours, but to the natural day of twenty-four hours, because various matters treated in tragedy, and even in comedy, are such as are more likely to happen in the night (adulteries, murders, and the like); and if it be said that night is naturally the time for repose, Segni answers that unjust people act contrary to the laws of nature.[181] It was about this time, then, that there commenced the historic controversy as to what Aristotle meant by limiting tragedy to one day; and three-quarters of a century later, in 1623, Beni could cite thirteen different opinions of scholars on this question.