Plots are either simple or complex, since the actions they
represent are naturally of this twofold description.
The
action, proceeding in the way defined, as one continuous
whole, I call simple, when the change in the hero's fortunes
takes place without Peripety or Discovery;
and complex,
when it involves one or the other, or both.
These should
each of them arise out of the structure of the Plot itself,
so as to be the consequence, necessary or probable, of the
antecedents. There is a great difference between a thing
happening propter hoc and post hoc.
Poetics. Aristotle. Ingram Bywater (Translator), ed. Gilbert Murray. Oxford. 1909.