Criticism
Aristotle lays out the terms in which poetry can be evaluated. From the outset, he recognizes that the standards of correctness and effectiveness that we apply to the language of poetry are not the same as those we use to evaluate other arts or even other uses of language–political speeches, for example. His approach to criticism is corresponds to his basic assumption that all art is mimetic; many of his remarks address the success of a work in representing perceived reality.

Faults in poetic works can be organized into two general categories: essential faults that impair the work as a while and accidental faults, such as factual errors, that may be irritating, distracting, and disappointing, but do not indicate a failure of artistic skill. Book XXV goes on to cover five sub-categories of faulty representation: impossibility, irrationality, moral harmfulness, contradiction, and failure to conform to artistic rules.

All errors should be avoided, Aristotle asserts, but he seems interested in establishing fair conditions for criticism. Throughout this chapter, he reflects on ways each of the five criticisms might be refuted, stressing that critics must always bear in mind the self-consistency and purpose of the work as a whole. Aristotle does not appear to address these errors in any particular order, and does not directly address the issue of moral harm. His remarks on the remaining categories are as follows:

Impossibility
Aristotle argues that poets should not, in general, depict impossible events, but that if the impossible event serves the artistic purpose, there is no fault in it (magical realist fiction might be a contemporary example of this kind of work). Aristotle reminds us that the "probable" is only "probable," and that in reality improbable events really do occur sometimes. Minor factual errors–representing a female deer as having antlers, for example–should not be grounds for dismissing the whole work.

Irrationality
Irrational behavior in a character can be justified in the same way as the impossible event: if it is necessary to the plot to have a character behave erratically or irrationally, such a representation is justified. Gratuitous badness in characters, however, should be condemned.

Contradiction When we meet with apparent contradictions in a work of art, we must apply careful methods of analysis to determine whether the word or phrase that seems to contradict an earlier expression in fact has the meaning we are assigning it. Aristotle suggests that we credit the author with enough intelligence to avoid blatant contradiction until we are convinced that the inconsistency is in fact a mistake and not an artistic strategy.

Failure to Conform to Artistic Rules
Before we conclude that a passage is poorly written or that a speech is unconvincing, we must examine the context of the passage in question to determine if this apparent fault serves some particular purpose in the work. A clumsy, unconvincing speech may perfectly suit the character who delivers it.

Careful attention to the way a poet is using language may reveal that an phrase or passage that appears confusing or nonsensical is in fact metaphorical, or intentionally ambiguous, or has some other function in supporting the representation.