On Jean-Paul Sartre's What is Literature? (1948), ch. 1 and 2.
What's the purpose of literature? Why write prose as opposed to poetry? Sartre was fending off criticism that his prose was too overtly political. Kant's view of art was still dominant, according to which good art is "disinterested," i.e., the spectator is supposed to appreciate the pure play of form. So if an artist is playing instead on your sentiments or your moral intuitions, that's impure, and likely cheap and manipulative.
In chapter one ("What is Writing?"), Sartre responds to this by distinguishing poetry from prose. In poetry (or painting or music), the focus is on the words (colors, sounds) themselves: How do the words sound? Prose, like any other kind of expository writing, uses words in their more usual manner, which is as signs that refer to things. What we care about in prose is communicating what we want to say, whereas in poetry, the point is entirely how you say it. To be sure, poetry is playing with words as signifiers, not as mere marks on a page (or sounds in the air), but language is in this case not being used. Sartre describes the poet as someone who refuses to merely use language, who pretends to be a stranger to ordinary language use in order to shift focus back from the signified ideas to the signifiers themselves.
In prose, on the other hand, style needs to serve the topic. You have to actually have something to say. Sartre is very critical of the cult of "great books" that embraces dead authors for their style while ignoring their practical exhortations (after all, what are we going to actually do in response to an ancient Greek author's opinion about the Peloponnesian War?), and then uses this as a template for how to treat living writers.
Mark, Wes, and Dylan spend a lot of time trying to get Sartre's philosophy of art straight, which is confusing and problematic: It would seem to pit him against all hybrids of poetry and prose, e.g., classical theater (Shakespeare and Sophocles are certainly filled with ideas and not just about the sounds of the words) and Lucretius (who is giving us poetry but expressly saying that the poetic form is just a tool to make the otherwise potentially boring ideas about physics and such go down easier).
And bringing in other art forms seems to make things more complicated. For example, poetry clearly differs from painting; zooming back from what a word signifies to the word itself is comparable to but interestingly different from zooming back from what a painted image depicts to the painting itself. Would photography then be the prose of image-making whereas painting is the poetry? This analogy helps undermine Sartre's claims in this work: Just as photography can be artistic, can be all about the image itself rather than about what is depicted (e.g., used for identifying a criminal through his image captured by a security camera), prose can be primarily artistic. The referential character of language makes it more able than any visual art for conveying ideas, but this will be just as true for poetry as for prose, as Sartre's own analysis of poetry in "Black Orpheus" makes clear. Perhaps, contra Sartre, artistically good prose is closer to poetry than it is to a treatise.