Castelvetro, however, was the first theorist to formulate the unity of place, and thus to give the three unities their final form. We have seen that Castelvetro's theory of the drama was based entirely upon the notion of stage representation. All the essentials of dramatic literature are thus fixed by the exigencies of the stage. The stage is a circumscribed space, and the play must be performed upon it within a period of time limited by the physical necessities of the spectators. It is from these two facts that Castelvetro deduces the unities of time and place. While asserting that Aristotle held it as _cosa fermissima e verissima_ that the tragic action cannot exceed the length of an artificial day of twelve hours, he does not think that Aristotle himself understood the real reason of this limitation.[191] In the seventh chapter of the _Poetics_ Aristotle says that the length of the plot is limited by the possibility of its being carried in the memory of the spectator conveniently at one time. But this, it is urged, would restrict the epic as well as the tragic fable to one day. The difference between epic and dramatic poetry in this respect is to be found in the essential difference between the conditions of narrative and scenic poetry.[192] Narrative poetry can in a short time narrate things that happen in many days or months or even years; but scenic poetry, which spends as many hours in representing things as it actually takes to do them in life, does quite otherwise. In epic poetry words can present to our intellect things distant in space and time; but in dramatic poetry the whole action occurs before our eyes, and is accordingly limited to what we can actually see with our own senses, that is, to that brief duration of time and to that small amount of space in which the actors are occupied in acting, and not any other time or place. But as the restricted place is the stage, so the restricted time is that in which the spectators can at their ease remain sitting through a continuous performance; and this time, on account of the physical necessities of the spectators, such as eating, drinking, and sleeping, cannot well go beyond the duration of one revolution of the sun. So that not only is the unity of time an essential dramatic requirement, but it is in fact impossible for the dramatist to do otherwise even should he desire to do so--a conclusion which is of course the _reductio ad absurdum_ of the whole argument.