Mark Granier accurately notes that in my post on the topic I failed to define what I meant by poetic difficulty. In order to clarify, I offer here my anatomy of difficulty. I present the several kinds of difficulty in order of ascending complexity and difficulty of resolution.
There is, first, lexical difficulty: the poem contains words with whose sense we are unfamiliar, or words used at variance from or even contrary to their dictionary definitions. Hart Crane’s poetry is a perfect example of such difficulty, full both of arcane and recherché words (“infrangible,” “transmemberment”) and of words given idiosyncratic or private meanings, as in the use of the word “calyx” to mean both a cornucopia (ironic, since the bounty is death’s) and “the vortex made by a sinking vessel” (Crane's explication) in “At Melville’s Tomb.”
Then there is allusive difficulty; the poem that alludes frequently eludes. The poet refers to something we’ve not heard of, assumes a piece of knowledge we don’t have. If one doesn’t know that Herman Melville wrote obsessively about the sea, then one won’t understand that the ocean itself is treated as his final resting place, though the man himself died on dry land. If one does not have “But at my back I always hear/Time’s wingèd chariot hurrying near,” and the rest of “To His Coy Mistress,” in one’s ear, the relationship of poem and title of Archibald MacLeish’s “You, Andrew Marvell” will appear rather opaque, and some of the poem’s sense of doom may be lost. Sometimes the allusion is only implicit: one will miss some of the force (and some of the humor) of Frost’s “For Once, Then, Something,” if one misses the presence of Narcissus in its description of a man who sees “Me myself in the summer heaven” reflected in the water of a well. Poems considered difficult often allude to material outside the common literary or intellectual frame of reference. Modernist poetry is particularly difficult in its wide range and idiosyncratic, often inexplicit, deployment of allusion.
There is also syntactical difficulty, the obstacle of complex, unfamiliar, dislocated, broken, or incomplete syntax: one cannot discern or reconstruct the relations of the grammatical units. Swerving away from the conventions of prose syntax has long been an integral part of poetic practice: as Howard Nemerov explains, it is “precisely the sort of rhetorical and musical variation which properly belongs to poetry and distinguishes it from prose” (“The Difficulty of Difficult Poetry,” in Reflexions on Poetry & Poetics). The long, Latinate sentences of Milton’s Paradise Lost are one example of this kind of difficulty; the fragmented, fractured syntax of much experimental poetry is another. In the case of Paradise Lost, one can parse the syntax with patience and careful attention; in many avant-garde poems, the syntax is intended to remain indeterminate, deliberately unparsable.
There is also semantic difficulty: we have trouble determining or deciding what a poem means, we cannot immediately interpret it. (It is important here to remember that sense and reference are distinct: sense is internal to the poem, as it is to language itself. As linguist David Crystal elucidates in How Language Works, “Sense is the meaning of a word within a language. Reference is what a word refers to in the world outside language.” From this perspective, it's more useful to think of the poem as a field of meanings than as a thing that means something else, a container for or vehicle of meaning.) Semantic difficulty encompasses figurative difficulty, in which we can’t unpack the poem’s metaphors, or can’t determine what is tenor and what is vehicle, especially when, as is frequently the case, one or the other is omitted, or when the presence and process of figuration is only implied. (This might be called the difficulty of elliptical figuration.) Difficulties interpreting tone, determining the stance and attitude the poem takes and wants the reader to take toward its material, would also fall under the heading of semantic difficulty.
Semantic difficulty can in turn be broken down into difficulty of explication and difficulty of interpretation. Some poems present both kinds of difficulty, some only one or the other. In the case of explicative difficulty, the reader cannot decipher the literal sense of the poem: “What is this poem saying?” One encounters this in Hart Crane’s “At Melville’s Tomb,” and he wrote an extensive explication of the poem for Harriet Monroe, then editor of Poetry. In the case of interpretive difficulty, one grasps what is being said on the literal level, but doesn’t know what it means, what it is meant to do. John Ashbery's poems, usually syntactically and explicationally clear, often present this interpretive difficulty. To say that one doesn’t know what a poem means, if one understands its literal sense, is to say that one doesn’t know why it’s saying what it’s saying. The reader asks, “Why am I being told/shown this?”
It is semantic difficulty which readers are usually experiencing when they say, “I don’t understand this poem.”
Then there is formal difficulty, what John Hollander calls the difficulty of problematical form: one cannot ascertain the poem’s shape, cannot hold it in one’s head as a construct. Or one cannot determine what kind of poem it is, and thus doesn’t know how to read it, in much the same sense that one might try and fail to “read” a person. The reader cannot determine or recognize the formal contract (on the analogy of Hollander’s concept of the metrical contract) to which the poem asks him or her to agree. This difficulty is most commonly encountered with poems that play with or violate conventions and expectations, that try to break and/or recreate form: remembering always the intimate relation of form and content, which, as Creeley wrote, are extensions of one another. The question the reader asks is, “What kind of poem is this?”
In the case of formal difficulty, one could add the possibility that the reader understands the terms of the poem’s formal contract, but refuses or feels unable to accede to them. Many American poetry readers today, raised on free verse, find it difficult to read metrical and/or rhyming poetry. They can’t hear its shape, can’t feel its rhythms. Ron Silliman has written that he can’t hear the rhythms of most British poetry. He can’t grasp its aural conventions; its sounds don’t make sense to his ear. This type of formal difficulty can be called rhythmic difficulty.
Formal difficulty is a particular case of what George Steiner, cited by Shetley, calls modal difficulty, my final kind. When we experience modal difficulty, “we fail to see a justification for poetic form, the root-occasion of the poem’s composition eludes or repels our internalized sense of what poetry should or should not be” (Shetley, After the Death of Poetry). Steiner actually writes, “what poetry should or should not be about,” but I broaden his statement to encompass not just topic or occasion but the poem’s status and recognizability as a poem. In the case of modal difficulty, a reader asks, “What makes this a poem?”
When people call a poem difficult, they are generally experiencing either semantic difficulty (“I don’t know what this poem is saying” or “I don’t know why this poem is saying what it’s saying”), formal difficulty (“I can’t see/hear the shape of this poem”), or modal difficulty (“I don’t recognize this as a poem”).
Another way to divide up the field would be to distinguish between difficulties of explication (which would include lexical, allusive, and syntactic difficulty), difficulties of interpretation (which would comprise the several varieties of semantic difficulty), and difficulties of recognition (which would encompass both formal and modal difficulty). These categories, of course, can and do overlap to a certain extent.
All of the kinds of difficulty I have enumerated and described are violations of readerly expectations. All readers, no matter how catholic in their tastes and in their knowledge, come to poems with some or another set of expectations. Readers may and do vary widely in their expectations of a poem, and they may have different expectations of different poems and different kinds of poems, but it’s impossible to approach a poem as if one were a Lockean blank slate. Every reader encounters poetic difficulty of some kind at some point.