What are the basic themes in the poetry of John Ashbery?

The importance of imagination as it determines how we live and who we are in the world is another recurrent and strong theme in Ashbery's poetry. For Ashbery, the imagination has a role not only in our dreaming and make-believe but in the interpretations we put on our own histories and on our sense of what is possible (both in terms of the possibilities of doing and the possibilities of being).

The imagination is, in a way, our power to determine our own meaning. 

In order for meaning to be brought to bear on any given instance, some creativity must be applied. Lacking imagination, facts remain inert. This thematic conceit is expressed almost directly in "Summer."

There is a sound like the wind

Forgetting in the branches that means something

Nobody can translate. And there is a sobering "later on," 

When you consider what a thing meant, and put it down.

Many of Ashbery's poems are reflective and retrospective, written in the past tense and in the form of a reverie or reminiscence. In this context, we can take note of the wildness that often characterizes the scenes Ashbery depicts. The past is not static. Rather, the past is a canvas that is painted retrospectively in the poem -- by the speaker of the poem. 

We might pose this technique as one related to questions of meaning and how meaning is constructed. Doing so would help to connect Ashbery to post-modernist writers concerned with the instability and fluidity of knowledge (which includes knowledge of the self). 

Another important aspect of the retrospective style in Ashbery's poetry is the implicit comment that this formula makes on identity. Using a collective "we" repeatedly in his poetry, Ashbery generates a sense that the poems speak both of and to specific groups of people and the poems lend insight into the meaning of what these groups did in the past, also offering added context by dint of the fact that these past episodes are being discussed in the present. 

Again, there is idea here that imagination must be applied to facts in order to for the facts to gain meaning. Ashbery's recollections of past episodes are not mere re-tellings of facts. They are creations and inquiries, often literally posing questions. 

[...] We must first trick the idea

Into being, then dismantle it, 

Scattering the pieces on the wind, 

So that the old joy, modest as cake, as wine and friendship

Will stay with us at the last, backed by the night

Whose ruse gave it our final meaning. 

(From "Flowering Death")

Looking at Ashbery's poetry with an eye to the role that imagination plays in the construction of meaning and in memory presents one compelling way to interpret many of his poems. Thematically aligned with post-modernism, this recurrent concept also speaks to the power of his poetry to evoke the familiar and complex romance of nostalgic identity (or the overlap of nostalgia and identity). 

Furthermore, the premise of the conceit that puts imagination at the center of meaning carries overtones that suggest a profound uncertainty or contingency of meaning and knowledge. If we must apply creativity in order to determine our own meaning (i.e., the meaning of our lives or the nature of our identity), then aren't we making ourselves up -- imagining ourselves?