A dream gives a poet permission to imagine what waking life won't allow, and Shelley uses that permission at once. "I dreamed that Milton's spirit rose" is a vision of the dead poet returning, not a statement of fact. What the vision shows is a poet's music doing the work of an army. Milton picks up an instrument, plays, and the structures of power begin to shake. The idea at the centre of the fragment is that poetry is not decoration but a moral and political force, strong enough to threaten thrones.
The second line packs in a good deal. Milton takes "from life's green tree his Uranian lute." The tree is green, so it is alive, and the lute is made from living wood, not a dead thing hung on a wall. "Uranian" points upward, to the heavens. The word recalls Urania, the muse Milton himself invokes in *Paradise Lost*, so Shelley dresses his predecessor in his own mythology. There is a quiet detail in "his touch," too. Milton went blind in later life, and the poem gives him a sense that needs no sight: he plays by touch, and the effect is enormous.
That effect is summed up in the phrase "sweet thunder," an oxymoron that holds the poem's whole claim. Sweetness belongs to music and thunder to storms and divine anger, and Shelley refuses to separate them. Beauty and force arrive together. The verb "flowed" makes the thunder liquid and gentle, yet the sentence turns on "and shook." That verb sits at the end of the line, and the sentence only completes itself in the next, "All human things built in contempt of man." The enjambment acts out the tremor: the reader is carried over the line break just as the structures are carried into collapse.
The target of the shaking is described with precision. These are things "built in contempt of man," institutions that exist against the people they claim to serve. Shelley then names them: "sanguine thrones," "impious altars," "Prisons and citadels." Together they cover government, religion, punishment and military power. Each phrase is loaded. "Sanguine" means blood-red, so the thrones are stained by the violence that holds them up. "Impious" turns the usual accusation around. Altars are supposed to be the most pious places, but here they are called irreverent, as if the real offence against the sacred were committed by those who use religion to prop up power. The mood is unmistakably radical, and Milton is the chosen champion.
The form deserves attention too. There is no rhyme. The lines are unrhymed iambic pentameter, or blank verse, which is the measure Milton used for *Paradise Lost*. The choice is neat: Shelley honours Milton by borrowing his own instrument. The rhythm is steady, with lines like "And sanguine thrones and impious altars quaked" falling into a firm, almost marching beat. Lacking rhyme, the poem sounds less like song and more like a declaration, and its weight rests on strong verbs (rose, took, flowed, shook, quaked) and the heavy nouns they act on. The rising and falling movement is also worth noticing. Milton's spirit "rose," and the thrones and altars "quaked." The poem moves upward in one clause and trembles downward in the next.
Then it stops. The final line, "Prisons and citadels...", is cut short, and the ellipsis leaves the sentence hanging. This is a fragment, and I won't guess at why it was left unfinished. But the break has a striking effect on a reader. The list of strongholds ends in mid-air, with the shaking still in progress. The poem never shows whether the buildings fall. It leaves them quaking, and it leaves us in the moment before the verdict.
That is part of what makes it worth reading. In six lines Shelley shows what he thinks poetry is for. It should unsettle the powerful, and one great poet can inspire another to do the same. The fragment also tells us something about the relationship between writers: Milton is not a museum piece here but a living presence whose music is still dangerous. Even unfinished, the poem is a small act of the thing it describes, a few lines of sweet thunder aimed at the walls.