since being unable to view it all at once, we lose the effect of a single whole; for instance, suppose a creature a thousand miles long.
As then creatures and other organic structures must have a certain magnitude and yet be easily taken in by the eye, so too with plots: they must have length but must be easily taken in by the memory.
The limit of length considered in relation to competitions and production [Note] before an audience does not concern this treatise. Had it been the rule to produce a hundred tragedies, the performance would have been regulated by the water clock, as it is said they did once in other days.
But as for the natural limit of the action, the longer the better as far as magnitude goes, provided it can all be grasped at once. To give a simple definition: the magnitude which admits of a change from bad fortune to good or from good fortune to bad, in a sequence of events which follow one another either inevitably or according to probability, that is the proper limit.
A plot does not have unity, as some people think, simply because it deals with a single hero. Many and indeed innumerable things happen to an individual, some of which do not go to make up any unity, and similarly an individual is concerned in many actions which do not combine into a single piece of action.
It seems therefore that all those poets are wrong who have written a HeracIeid or Theseid or other such poems. [Note] They think that because Heracles was a single individual the plot must for that reason have unity.
But Homer, supreme also in all other respects, was apparently well aware of this truth either by instinct or from knowledge of his art. For in writing an Odyssey he did not put in all that ever happened to Odysseus, his being wounded on Parnassus, for instance, or his feigned madness when the host was gathered(these being events neither of which necessarily or probably led to the other), but he constructed his Odyssey round a single action in our sense of the phrase. And the Iliad the same.
As then in the other arts of representation a single representation means a representation of a single object, so too the plot being a representation of a piece of action must represent a single piece of action and the whole of it; and the component incidents must be so arranged that if one of them be transposed or removed, the unity of the whole is dislocated and destroyed. For if the presence or absence of a thing makes no visible difference, then it is not an integral part of the whole.
What we have said already makes it further clear that a poet's object is not to tell what actually happened but what could and would happen either probably or inevitably.
The difference between a historian and a poet is not that one writes in prose and the other in verse—