To Althea, from Prison Freedom and Confinement Quotes
How we cite our quotes: (Line)
When flowing cups pass swiftly round
With no allaying Thames (9-10)
The "flowing cups" seem like a metaphor for freedom. First off, they flow, and they're also not diluted, not "confined" or "inhabited" by any sort water (the "allaying Thames").
When, like committed linnets, I
With shriller throat shall sing
The sweetness, mercy, majesty,
And glories of my King; (17-20)
These lines mimic the speaker's confinement. Lots of letters are repeated (notice the alliteration of the words that begin with s and m), which suggests that—even on a formal level—the speaker's choice of letters is "committed" or "imprisoned" as well.
Stone walls do not a prison make,
Nor iron bars a cage; (25-26)
If "iron" and "stone" don't make "prisons," what does? The speaker suggests that the only real prisons out there are ones people make themselves. The human mind is more powerful than strong building materials like iron and stone.