The Poetics of Aristotle

Plots are either Simple or Complex, for the actions in real life, of which the plots are an imitation, obviously show a similar distinction.  An action which is one and continuous in the sense above defined, I call Simple, when the change of fortune takes place without Reversal of the Situation and without Recognition.

A Complex action is one in which the change is accompanied by such Reversal, or by Recognition, or by both.  These last should arise from the internal structure of the plot, so that what follows should be the necessary or probable result of the preceding action.  It makes all the difference whether any given event is a case of propter hoc or post hoc.