In his definition of tragedy Aristotle says that the play must be complete or perfect, that is, it must have unity. By unity of plot he does not mean merely the unity given by a single hero, for, as he says, "infinitely various are the incidents in one man's life which cannot be reduced to unity; and so, too, there are many actions of one man out of which we cannot make one action. Hence the error, as it appears, of all poets who have composed a Heracleid, a Theseid, or other poems of the kind. They imagine that as Heracles was one man, the story of Heracles ought also to be a unity."[177] This is Aristotle's statement of the unity of action. But what is the origin of the two other unities,--the unities of time and place? There is in the _Poetics_ but a single reference to the time-limit of the tragic action and none whatsoever to the so-called unity of place. Aristotle says that the action of tragedy and that of epic poetry differ in length, "for tragedy endeavors, so far as possible, to confine itself to a single revolution of the sun, or but slightly to exceed this limit; whereas the epic action has no limits of time."[178] This passage is the incidental statement of an historical fact; it is merely a tentative deduction from the usual practice of Greek tragedy, and Aristotle never conceived of it as an inviolable law of the drama. Of the three unities which play so prominent a part in modern classical drama, the unity of action was the main, and, in fact, the only unity which Aristotle knew or insisted on. But from his incidental reference to the general time-limits of Greek tragedy, the Renaissance formulated the unity of time, and deduced from it also the unity of place, to which there is absolutely no reference either in Aristotle or in any other ancient writer whatever. It is to the Italians of the Renaissance, and not to the French critics of the seventeenth century, that the world owes the formulation of the three unities. The attention of scholars was first called to this fact about twenty years ago, by the brochure of a Swiss scholar, H. Breitinger, on the unities of Aristotle before Corneille's _Cid_; but the gradual development and formulation of the three unities have never been systematically worked out. We shall endeavor here to trace their history during the sixteenth century, and to explain the processes by which they developed.