You may be an ambassador to England or France
You may like to gamble, you might like to dance
You may be the heavyweight champion of the world
You may be a socialite with a long string of pearls.
But you're gonna have to serve somebody, yes indeed
You're gonna have to serve somebody,
It may be the devil or it may be the Lord
But you're gonna have to serve somebody.
- Bob Dylan
No one can serve two masters; for either he will hate the one and love the other, or else he will be loyal to the one and despise the other. You cannot serve God and Mammon.
- Gospel of Matthew (Sermon on the Mount)
Around 1930, when Stalinist repression was closing in around poet Osip Mandelstam, he was asked by an interviewer for a definition of poetry. He replied, in typical pithy fashion : "the poet's sense of being right" (or sense of "inner rightness").
M's adage planted itself in my mind years ago, & keeps returning, like an unsolved puzzle. About 13 years ago I wrote a couple of impressionistic essays for Chris Reiner's spunky little magazine Witz, which attempted to come to terms with it; I will try to come up with an online copy of the 2nd one ("Sense of Being Right"), which raised more questions than it answered. (The 1st Witz essay can be found here, & here.)
The crux of the puzzle is that M.'s defining characteristic for poetry comes not from aesthetics but from ethics and morality. In the latter Witz essay, I wrote that M. had asserted that poetry's purpose - its end, its telos - resided beyond itself : in a realm of the moral imperative, an ethical Absolute. "Rightness". And I said that it was a mistake to separate this moral telos from some hypothetical (aesthetic) beginning - in creative nature, in process or praxis, etc. Rather, M. is saying that the "beginning" is in the end : they cannot be separated. And he bolsters this position by suggesting that poetry (at least, Acmeist poetry - his poetry) has, as its subject, "the idea of Man". Not Man as Citizen (or subject of history or the State), but the idea of Man in its most universal and inclusive (& non-gender-specific!) sense :
"It's not Rome the city that lives through the centuries
But man's place in the universal scheme."
In other words : our human being, our nature as individuals and as species, is constituted and characterized by moral conscience. Our consciousness is a moral consciousness : our inner compass or gyroscope, our "sense of being right", is an expression of inherent justice (or the seeking thereof).
Mandelstam elaborates on this in another essay, on Pyotr Chaadev, a 19th-cent. thinker perhaps somewhat comparable to Emerson in the U.S. Chaadev's theme was that Russia's destiny depended on a quality (represented by the choices & acts of individual Russians) which he called "moral freedom". Roughly speaking, he was reminding Russians that individual conscience - and not the forces of the collective, the State, or history - is the anchor of civilization.
In another place, M. portrayed this general view of things as "the gold coins of humanism" - a Renaissance-vision of civilization as a kind of personalized, human hearth. Nothing is outside the human spirit, which makes the world a "home" ("domestic hellenism") : and yet Man is the measure of all things only so long as she/he remains devoted to a consciousness of rightness & justice - to an absolute, standing beyond our willful grasping. To a "plumbline", in other words. So this allegiance to something beyond ourselves is actually our defining, distinguishing characteristic - that which makes us human.
What I am suggesting here, is that our "plumbline" is not simply a kind of critical tape measure, a tool we can apply in some detached technocratic fashion. In Mandelstam's view, poetry is a living tradition, a distinct emanation of our humanness per se : it is far too engaged and intertwined with the moral & existential choices facing individuals and peoples, now & every day, to support the arrogance of mandarins, aesthetes, jobbers, technocrats. Poetry is the living speech of the Personal and the Human - the Individual and the People; that which reaches deep, grasping & mirroring our common nature with a piercing, proverbial intensity.
& if culture, civilization & poetry are built on the expressions of individual conscience & spiritual freedom, then, for working poets, issues of theme, rhetoric, & mode of address become both problematic & more pressing. None of the partial measures of value - populism, programmatic engagement, aesthetic autonomy, style for its own sake, canonicity, etc. etc. - can be deemed either absolute or even substantial. Rather, the measure of value seems to return to a kind of independent affinity of whole persons - a line of ethical/aesthetic communication - established between poet and reader, dramatist and audience. Here the ethical can be distinguished (analytically) from the aesthetic - but they cannot be separated. Poetry returns to humane expression between integral and individual persons. Concrete & unique, because moral-historical existence is only apprehended in the incomparable particulars of changing life.
Thus we are reminded that a plumbline dangles from a point of leverage (related, perhaps, as Joseph mentioned in a comment, to Pound's Confucian notion of the "unwobbling pivot") which stands in its own firmament, and firmly beyond the reach of those changeable circumstances to which it is applied as measure. This is an image of human conscience aligned with justice, the moral absolute. The fact that both poetry and human life generally dangle from the same spiritual pivot is what makes possible the various syntheses, on various levels (ethical, aesthetic, political), of the mediating "golden mean". And this same pivotal situation holds out a perennial promise to the artist, that her labors will find an echo with an understanding audience.