Poetics (7.4-7.8)

A beginning is that which is not a necessary consequent of anything else but after which something else exists or happens as a natural result.

An end on the contrary is that which is inevitably or, as a rule, the natural result of something else but from which nothing else follows;

a middle follows something else and something follows from it.

Well constructed plots must not therefore begin and end at random, but must embody the formulae we have stated.

Moreover, in everything that is beautiful, whether it be a living creature or any organism composed of parts, these parts must not only be orderly arranged but must also have a certain magnitude of their own;