Aristotle argued (according to notes from his lectures, later collated) that there are three distinct kinds of speech or literature. The first, the Dialectic, is the language of logic, metaphysics, physics, philosophy. It is language deployed in the search for truth. Through dialectic antithesis is applied to thesis in a Socratic way, producing Synthesis: superior truth. This is the foundation of the process by which Reason is applied in comprehending the ‘verse, and expressing truths about it.
Rhetoric is the second mode of speech, subjected to analysis in the Rhetoric of Aristotle. It is ‘the counterpart of dialectic’, the art of making use of all of the available means of persuasion. So rhetoric is concerned with persuasion rather than truth, with probability rather than Absolutes. Rhetoric is the language of law and politics and forensics.
The third kind of speech, or language, is the Poetic. Because it is distinct from either the Dialectic or the Rhetoric, Poetic language is by definition divorced from both truth and persuasion.
Plato said he would ban poetry from his ideal society. Aristotle said the proper function of poetry is adornment and flattery.
This, then, is the bone of contention: my argument is that fiction (not quite the same thing as poetry, granted) does have both rhetorical and philosophical function, and that Aristotle was wrong to relegate this mode of language to a tertiary tier (where, perhaps, it could do no harm). The polemic here, in dramatic terms, is that a simplistic approach to the problems flowing from language to perception of truth is no approach at all. The division of language into three registers or keys has constrained all the forms of language, weakening their import and utility.
see also: a definition for fiction