A friend of mine pointed out that semi-recent posts on decorum are a bit of a muddle and that I could have clarified more of what I meant by Arnold’s Platonism. The two are related, and I will explain how here. The post was written as a letter to a friend, but who wants to receive as dry and explicatory a letter as this:

A very quick note just because it keeps coming up–over the posts on decorum, I was evolving an idea, and think I can put it neatly now. The word gained meanings because it’s both ideologically charged, neutral, and specific to literary history, and I was struggling to work them out.
Decorum is language appropriate to the conditions of its utterance. That’s the neutral sense. That means that poetry (anything with “style”) needs to possess decorum: it is well-judged for the conditions of utterance that it implies (it gets the conditions of the judgment within the judgment, and it meets those conditions).
But then there’s also the snob/polite society sense. “Appropriate” becomes ideologically charged: “as a gentleman” would find it appropriate, as “someone of the white middle class would find it appropriate.”
For some poets (Davie’s “urbane” poets of the 18th century), the two senses overlap; for others, there is a tension or clash between the two sorts (Dickinson; Ginsberg).
Then there’s a third possibility, of style: low, middle, high; base, humble, elevated; whatever other categories. In this case, the appropriateness of an utterance will be judged in terms of a broader style, a broader region or pattern of verbal use that might be low, middle or elevated.
For Racine, decorum has to do with the presence, in French, of a fairly coherent and thickly articulated (largely thanks to him) “elevated style”.
For Burns, the interplay of low and humble styles matters, but depends upon those distinct categories existing.
Those different registers can themselves be social and class-based: Racine’s royal and mythic heroes deserve the high style. They can also be regional (Burns’ scots). They can also have to do with the magnitude or seriousness of the subject matter: Milton wants a highs style.
But high style doesn’t need to be florid or artificial. It can also be severe: “Samson Agonistes” or “Michael” by Wordsworth, or the “Ode.” There is more than one form of elevation, in other words.
Tennyson, I think, wanted to create an elevated style that was especially harmonious.
Different poets will have different motives for writing in a low, middle, or high style, and ideology and class is certainly one of them. But also, bound up in ideology and class, is a matter of historical or temporal perspective. The elevated style can accompany a lofty perspective on the whole of history, or even a timelessness, a timeless standard by which the actions of a scene are judged. The middle puts one into the register of realism, of daily life, of habit; the low style, because of its incorporation of slang, because often attached to figures of little historical value, as is commonly thought, is more caught up in the ephemeral.
Likewise, it might be thought that each register is appropriate to a different bodily experience, a different experience of the bodily. Keats’ boldness might be thought of as his incorporating more sensuality into a high style, or of moving into a high style through sensuality.
What is “appropriate” or decorous in these cases could be said to be knowing what is elevated, what is middle, and what is low; having a sense of these styles can itself make decorum possible in another sense, that might be more or less obviously dependent on class or other identity marker.
Having recourse to decorum in terms of register means having recourse to register itself, which seems (the awareness itself) something increasingly difficult for poets; the loss would seem to involve not only an awareness of what language can do, and be, as itself, but also vantage points on time and the body. The loss, in other words, is real.
The sensitivity of novelists to these matters is quite different, and will often involve in how the narrator aspires to decorum, to a particular register, or contains them with clashes or transitions, or simply mediates between them.
The relevance to Arnold’s notion of “idea” is obscure but intuitive: the “idea” of seriousness is not a theory or ramified thought, but something affective and nonetheless defined, much like a Platonic ideal. In the Renaissance, Hermogenes uses “idea” in “Idea of Style” to mean Platonic ideal, the thought being that different stylistic qualities–sweetness, strength, beauty, severity, eloquence–represent different Platonic ideas realized in language. Arnold wants for “high seriousness” to not, in itself, be an idea, but to represent the successful application of a lofty idea, of truth perhaps, and, in his touchstones, offers a gallery of applications, some eloquent and others severe. There’s no evidence that Arnold had Plato in mind when he talks about the application of ideas, but the absence of many other explanations, and the meanings and history inherent in the word, as well as Arnold’s education and era, suggest that Plato might have been present in the word, whether Arnold fully knew or meant it.
It makes sense intuitively to me for the reason that the gap between conception and realization of a work of art is a gap between an unrealized idea and something that is more than and other than the idea: the idea applied to life. But “idea” in that sense cannot just be a draft or plan, refers to something hazier, a “take,” a “vision,” but without content in focus or form even planned out. As with Plato’s metaphysics more generally, the theory of forms is not a theory of what lies behind what we say and see, but a theory of what sort of terms and notions we need if we are to say certain things at all–things about how we know, or how we create. Arnold is trying to say what happens we encounter that strange phenomenon of the greatest poetry–something far off made to seem reflected in the world we know. For most of us, we never have an original idea; others, less lucky in some ways, more in others, the idea is visible but the application impossible; then there are the lucky few who make us see that there is an idea to be applied at all, so that what happens in the world is seen under its auspices, participating in it.