Wallace Stevens, from "An Ordinary Evening in New Haven"

XII

The poem is the cry of its occasion,
Part of the res itself and not about it.
The poet speaks the poem as it is,

Not as it was: part of the reverberation
Of a windy night as it is, when the marble statues
Are like newspapers blown by the wind. He speaks

By sight and insight as they are. There is no
Tomorrow for him. The wind will have passed by,
The statues will have gone back to be things about.

The mobile and immobile flickering
In the area between is and was are leaves,
Leaves burnished in autumnal burnished trees

And leaves in whirlings in the gutters, whirlings
Around and away, resembiling the presence of thought
Resembling the presences of thoughts, as if,

In the end, in the whole psychology, the self,
the town, the weather, in a casual litter,
Together, said words of the world are the life of the world.

...

XXVIII

If it should be true that reality exists
In the mind: the tin plate, the loaf of bread on it,
The long-bladed knife, the little to drink and her

Misericordia, it follows that
Real and unreal are two in one: New Haven
Before and after one arrives or, say,

Bergamo on a postcard, Rome after dark,
Sweden described, Salzburg with shaded eyes
Or Paris in conversation at a café.

This endlessly elaborating poem
Displays the theory of poetry,
As the life of poetry. A more severe,

More harassing master would extemporize
Subtler, more urgent proof that the theory
Of poetry is the theory of life,

As it is, in the intricate evasions of as,
In things seen and unseen, created from nothingness,
The heavens, the hells, the worlds, the longed-for lands.

~

From The Auroras of Autumn (1950)

One could hardly talk about the theme of ars poetica without quoting Wallace Stevens (1879-1955). Arguably, the entirety of Stevens' writing is about poetry itself - or, more widely, about the relationship between the imagination and reality. I could have picked any of a hundred poems from his collected works, as you can tell simply from the titles of some of the other major contenders: "The Idea of Order at Key West", "Poetry is a Destructive Force", "The Poems of Our Climate", "Of Modern Poetry", "Men Made Out of Words", "Notes toward a Supreme Fiction", "The Solitude of Cataracts", "The Ultimate Poem Is Abstract", "A Primitive like an Orb", "The Plain Sense of Things", "The Planet on the Table", "Not Ideas about the Thing but the Thing Itself" etc. etc. etc. But I chose these sections because his later, meditative mode is often undervalued, and certainly less anthologized than his earlier works.
This later moder finds Stevens pursuing the style and form of, as he calls it in "Of Modern Poetry", "The poem of the act of the mind." Not only is the poem not paraphrasable, but it is also not seperable from the experiences of composition - this is a poetry of process, the very process of the mind encountering "reality," which is to say all that we can know: "Part of the res itself and not about it," "words of the world are the life of the world." Perception, rather than being unreal, is reality for Stevens: "reality exists / In the mind ... Real and unreal are two in one". Poetry, then, being perception/imagination/the mind in process, is not an imitation of reality, but is real experience itself: "the theory / Of poetry is the theory of life, // As it is, in the intricate evasions of as, / In things seen and unseen, created from nothingness, / The heavens, the hells, the worlds, the longed-for lands"—not mere physical "reality," but life in its lived fullness.

(More about Wallace Stevens, including audio: www.poets.org/wstev/)