Aristotle was writing about tragedy.  He defines tragedy as "the representation of an action that is complete and whole and of a certain amplitude."  The idea that a tragic plot should be "complete and whole" is fundamental.  Here's what he means by "whole":
 

"Now a whole is that which has a beginning, a middle and an end.  A beginning is that which does not necessarily come after something else, although something else exists or comes about after it.  An end, on the contrary, is that which naturally follows something else as a necessary or as a usual consequence, and is not itself followed by anything.  A middle is that  which follows something else, and is itself followed by something.  Thus well-constructed plots must neither begin nor end in a haphazard way, but must conform to the pattern I have been describing."

A good essay also has this quality of "wholeness".
 
Aristotle, On the Art of Poetry.
Translated by T.S. Dorsch
Harmondsworth: Penguin Books, 1965
page 41.