Part of the complex of topics out of which the "Plumbline" has started to emerge has to do with the relation between poetry and drama. (This emergence was actually triggered by an anecdote about Abraham Lincoln reading Macbeth.)
Again, one of the further motivating impulses has been a reaction, on our part, against various kinds of stylistic excess or extremism, which tend to inhibit the range of poetry's representations - that is, poetry's ability to marshall its medium (language) in the cause of presenting some recognizable scene, character, or action from nature or experience.
Finally, a third impulse or direction taking shape here has to do with the relation between criticism & poetry, theory & practice. This is a question we have to ask ourselves : how do we frame a critical approach which is actually useful and relevant, both to our own writing, and to the way we read & evaluate the poetry of others? The "plumbline", running down the center, is a kind of symbol for analysis : the discovery & application of some kind of measure (an analytical tool), which would help us distinguish the different parts of a good poem, and how they work together.
All three of these concerns come together in the work of the mid-20th-cent. scholar R.S. Crane, a member of the so-called Chicago School of literary critics. If I were to choose one book to serve as a guide to the "plumbline", it would be Crane's The language of criticism and the structure of poetry (Univ. of Toronto Press, 1953). Sad to say, this book is out of print & not so easy to come by. And his thinking is so meticulous, judicious, & rich, that it will be a real challenge for me to try to summarize and adapt it to our use.
For now, anyway (I'm at work!) - here is a very quick sketch of some aspects of Crane's approach that seem especially relevant:
1. Crane tries very hard to defuse the polemical power of competing critical approaches. He calls for a kind of critical "relativism". It's not that there is no such thing as the truth : rather, there is no such thing as a single right method of criticism, since poets, critics & readers all come with a great diversity of interests, motives, needs & approaches.
2. Aligned with #1 is Crane's allegiance to Aristotle. Much of the book is an investigation and adaptation of Aristotle's empirical, analytical approach in the Poetics. Aristotle's own careful definitions of his "lines of inquiry" limit a priori generalizations, theoretical abstractions : he starts from the particular object at hand, and develops a language and method of analysis specifically drawn from that object itself. Thus the Poetics is a practical criticism, in that Aristotle is asking, "what is a good poem? & how do poets make them?"
3. The results of such a method of empirical, differential analysis are very striking. Crane, following Aristotle, finds that poems are "concrete wholes". That poems are formal shapes, or structures, whose holistic or organic design is not reducible to the analysis of their grammar or rhetoric. That the fundamental, distinguishing characteristic of the specific kinds of poems which Aristotle discusses - that is, "imitative" or mimetic poems - is just that : they are representations of persons, actions or things as we know them in life, from nature. Crane makes it clear : this is not the only kind of poem there is. In fact, many great poems (such as Dante's Divina Commedia, Spenser's Faerie Queene) are of a different kind altogether. They are not mimetic : rather, they are didactic, based not in imitation, but in argument and persuasive rhetoric.
4. The mimetic poet shapes all the elements of the work - including setting, character, plot, thought, style - in order to effect an integral or holistic impression. This is the goal of the "shaping process" which is composition.
One can see immediately how Crane's Aristotelian distinction of basic kinds of poetry (mimetic, didactic) relates to issues we have been discussing. For example, the distinction between the metaphysical & the Restoration poets, as abstracted by Johnson & Eliot, may be seen to hinge on this distinction between mimetic & didactic. The question of finding a "happy medium" folds into Crane's description of the shaping process of a mimetic whole.
Here's a brief quote from Crane, which bears on another area of interest : the issue of beauty as proportion - as the finding of a point of mediation, which allows for a synthesis of distinctive parts, the composition of an integral whole. This is from a passage where Crane is talking about the critical process as a comparison between the hypothetical aims and intentions of the poet, and the "necessities and possibilities" inherent in the hypothetical form of that particular kind of work. (I emphasize "hypothetical" because, as Crane points out, we can never know for certain about the poet's intentions, or about the exact structure or form she/he is attempting to compose.)
"There is nothing unfair to the writer in such an approach, inasmuch as we are not engaged in a judicial process of bringing the work under a previously formulated general theory of literary value, but in a free inquiry whose aim is simply the discovery of those values in [his/her] work - among them, we always hope, unprecedented values - which [she/he] has been able to put there. They will always be values incident to the form of the work and its matter at all of its structural levels; and it will be appropriate to interpret what we find in terms of a distinction between three classes of works considered from this point of view : works that are well conceived as wholes but contain few parts the formal excellence of which remains in our memory or invites us to another reading; works that are rich in local virtues but have only a loose or tenuous overall form; and works that satisfy Coleridge's criterion for a poem, that it aims at 'the production of as much immediate pleasure in parts, as is compatible with the largest sum of pleasure in the whole.' These last are the few relatively perfect productions in the various literary kinds, and as between the other two we shall naturally prefer the second to the first." [pp. 182-183]
Taken out of context, this sounds like the dullest & driest kind of study outside of Economics. But in the context of an investigation of actual poems, along "lines of inquiry" such as Aristotle (& Crane) suggest, this passage has a lot of useful implications.
There is the notion that a beautiful or "perfect" work will naturally remain in memory, & be something to which we want to return. & this in turn is related to the Aristotelian concept of imitation, that a (mimetic) poem presents an image from experience which we recognize, which we can possibly identify with on some level, and which therefore moves & pleases us by its truthful mirroring.
Further, there is the assertion, in the quotation from Coleridge, that a good poem will exhibit a harmony betwen whole and part, such that distinct parts give pleasure, but do not detract from the pleasure of the whole. This is exactly where we were tending, in the discussions of the plumbline as "golden mean", & etc.
Finally, there is the implication that - through a careful, empirical investigation of particular poems, looking toward what aims the poet had in composing its particular features in just that way, and asking if the overall form or dynamic of the work has achieved those aims - we can come to a fair critical estimate or understanding of a poem's import and achievement. In other words, a kind of conscious or reflective intellectual sanction for our initial, instinctive responses. (We like a poem : now we can say why.) This seems to be a process which we would try to approximate in the kind of aesthetic measuring that a "plumbline' suggests.