Idea: The unity of a plot does not consist in having one man as its subject. For example, “an infinity of things can happen” to one man, but that doesn’t mean there is a unity to the action (1451a 16ff). Since poetry (e.g., literature) is an imitation of action, it must represent one action and the several incidents attendant to it.
The poet must describe what is possible or necessary but not what just happened, otherwise he would be a historian. Poetry must deal with universals. The story or plot should drive the verses (or style) rather than the other way around. An episodic plot, which is a poor one, lacks any probability of necessity in the sequence of its actions.
Definition of Tragedy: an otherwise just, if average man, has an error of judgment and the world comes crashing down on him. The hero must go from happiness to misery.
“Poetry demands a man with a special gift for it, or else one with a touch of madness in him.”
“A likely impossibility is always preferable to an unconvincing possibility.”
Logical Investigations 