The possesses certain formal or structural features

The subject of the Poetics is poetry, including epic poetry, tragedy and comedy. Unlike Plato, Aristotle regards poetry as a techne. The practice of poetry is governed by rules; these rules can be formulated and taught. Poetry is rationally comprehensible.
The rules for a genre of poetry can be derived from examination of individual examples of that genre, eg, tragedy. The aim is to see what, eg, tragedies, have in common.
Aristotle agrees with Plato that epic poetry, tragedy and comedy are essentially mimetic. But he decisively rejects Plato’s view that mimesis in art consists in copying from life.
According to Aristotle, mimetic poetry seeks to offer a convincing semblance of life. It seeks to be true to life without copying from it. Moreover, the literary work of art possesses certain formal or structural features which differentiate it from a mere mirror image of life.
The most important of these formal requirements is unity. Taking tragedy as an example, the representation of the action of the play is the plot, and the plot is the ordered arrangement of the incidents. A well-constructed play must have a beginning, middle and an end. The plot must neither begin nor end in a haphazard way. The plot of the play must represent the action of the play as a unified whole. ‘Incidents must be so arranged that if any of them is differently placed or taken away the effect of wholeness will be seriously disrupted.’ (Poetics, ch 8).
It is the historian who makes a copy or record of things that have happened. The poet describes what might happen. The kinds of things that might happen are those which, in the circumstances, are either probable or necessary.
Poetry is concerned with universal truths; history treats of particular facts.
Universal truths in this context refer to the kinds of things that a certain type of person will probably or necessarily do or say in a given situation.
In order to produce the pleasure that is appropriate to tragedy, the plot itself must awaken fear and pity in the audience. In this way tragedy effects a healthy catharsis (purgation) of these emotions. Thus, Aristotle implicitly rejects Plato’s view that the performance of a tragedy has a bad effect on the character of the audience.