Aristotle never refers to Plato directly , but much of his Poetics is a covert reply to his master. They are of different temperaments : Plato is a transcendentalist with an artistic temper, Aristotle is an organized scientist who arrives at his principles through observation and analysis. Plato is an idealist; Aristotle is  a compromiser who reconciles the real and the ideal. Imitation means servile copying to Plato ; Aristotle regards it as a creative process and breathes a new life and soul into it, saying that poetry is concerned not with what is but with should be, and it gives an idealized version of reality. The truth of history is the truth of reality, the truth of poetry is the truth of ideal. Plato compares poetry to painting ; Aristotle , to music.

Aristotle's poetics is concerned chiefly with tragedy and epic . either he did no write  on other literary types or those parts are lost. He prefers tragedy to epc because the former I acted while the latter is narrated. He defines tragedy as ' imitation of action " which is " serious , complete ,and of a certain magnitude" . the action must be complete ; that is , it has a beginning ,a middle ,and an ending, it is unified. The chorus is an integral part of the plot , and any kind of violence should be reported , not o the stage the denouement or unraveling of the plot should be a natural consequences of incidents.

Aristotle prefers the complex plot to the simple plot. A complex plot is one in which the change of the fortune is accompanied by peripeteia ( the reversal of intention) or Anagnoresis ( recognition or discovery) or by both. The best plot is in which the action is perpetuated and then comes the discovery.  The action should also follow the laws of probability and necessity . it should be probable , possible, convincing and believable.

Aristotle believes that the plot is the most important part of the tragedy. Then comes characterization. Comedy is an imitation of characters of a lower type; tragedy is an imitation of characters of a type. Characters must be good, have propriety , be true to life and consistent. The tragic hero , besides having these qualities , should be a distinguished person who is neither innocent nor vicious; he is a rather good man with a tragic flaw (hamartia) . since the aim of tragedy is to achieve catharsis by arousing pity and fear, the representation of innocent and wicked protagonists achieves no tragic effect because the change of an innocent man's fortune from happiness to misery is painful , not tragic , and the fall of a wicked man is not  also tragic although it satisfies the moral purpose.  

The emphasizes of tragedy is catharsis (purgation) of undesirable emotions of pity and fear. Catharsis leads to emotional balance which is health-giving and we take pleasure because we have taken part in the action on the stage without being destroyed . the effect of catharsis  on the soul  is like that of medicine on the body; it produces a condition of emotional balance .

Aristotle emphasizes the unity of action, requires the unity of time and implicitly demands the unity of place. " tragedy lives within a single evolution of the sun" Unity of place confines the action to a single place. Aristotle's emphasis on rules and principles of art makes it clear that to him art, education, training , or labor is required . it doesn't mean that he is unaware of genius ( nature-talent). In fact , his view of mimesis demands poetic talent and imaginative power , what Plato calls poetic madness.

Source:

A Survey of English Literature (I) by Dr. Abjadian