I have added a bit of context and highlighted in boldface the specific passages that the discussion focuses on.
My God! Can it be possible I have
To die so suddenly? so young to go
Under the obscure, cold, rotting, wormy ground!
To be nailed down into a narrow place;
To see no more sweet sunshine; hear no more
Blithe voice of living thing; muse not again
Upon familiar thoughts, sad, yet thus lost!
How fearful! to be nothing! Or to be--
What? Oh, where am I? Let me not go mad!
Sweet Heaven, forgive weak thoughts! If there should be
No God, no Heaven, no Earth in the void world--
The wide, gray, lampless, deep, unpeopled world!
Isabella, a nun, can save her brother's life by yielding her chastity to the wicked ruler -- which she refuses to do. Here she brings Claudio the news that he cannot expect a reprieve.
CLAUDIO: Has he affections in him,
That thus can make him bite the law by the nose,
When he would force it? Sure, it is no sin,
Or of the deadly seven, it is the least.
ISABELLA: Which is the least?
CLAUDIO: If it were damnable, he being so wise,
Why would he for the momentary trick
Be perdurably fined? O Isabel!
ISABELLA: What says my brother?
CLAUDIO: Death is a fearful thing.
ISABELLA: And shamed life a hateful.
CLAUDIO: Ay, but to die, and go we know not where;
To lie in cold obstruction and to rot;
This sensible warm motion to become
A kneaded clod; and the delighted spirit
To bathe in fiery floods, or to reside
In thrilling region of thick-ribbed ice;
To be imprison'd in the viewless winds,
And blown with restless violence round about
The pendent world; or to be worse than worst
Of those that lawless and incertain thought
Imagine howling: 'tis too horrible!
The weariest and most loathed worldly life
That age, ache, penury and imprisonment
Can lay on nature is a paradise
To what we fear of death.
"The juxtaposition is enough to expose the vague, generalizing externality of Shelley's rendering. Claudio's words spring from a vividly realized particular situation; from the imagined experience of a given mind in a given critical moment that is felt from the inside -- that is lived -- with sharp concrete particularity. Claudio's 'Ay, but to die ...' is not insistently and voluminously emotional like Beatrice's ('wildly')
O My God! Can it be possible ...but it is incomparably more intense. That 'cold obstruction' is not abstract; it gives rather the essence of the situation in which Claudio shrinkingly imagines himself -- the sense of the warm body (given by 'cold') struggling ('obstruction' takes an appropriate effort to pronounce) in vain with the suffocating earth. Sentience, warmth and motion, the essentials of being alive as epitomized in the next line, recoil from death, realized brutally in the concrete (the 'clod' is a vehement protest, as 'clay,' which 'kneaded' nevertheless brings appropriately in, would not have been). Sentience, in the 'delighted spirit,' plunges, not into the delightful coolness suggested by 'bathe,' but into the dreadful opposite, and warmth and motion shudder away from the icy prison ('reside' is analogous in working to 'bathe'). The shudder is there in 'thrilling,' which also -- such alliteration as that of 'thrilling region' and 'thick-ribbed' is not accidental in a Shakespearian passage of this quality -- gives the sharp reverberating report of the ice as, in the intense cold, it is forced up into ridges or ribs (at which, owing to the cracks, the thickness of the ice can be seen).
But there is no need to go on. The point has been sufficiently enforced that, though this vivid concreteness of realization lodged the passage in Shelley's mind, to become at the due moment 'inspiration,' the passage inspired is nothing but wordy emotional generality. It does not grasp and present anything, but merely makes large gestures towards the kind of effect deemed appropriate. We are told emphatically what the emotion is that we are to feel; emphasis and insistence serving instead of realization and advertising its default. The intrusion of the tag from Lear brings out the vague generality of that unconscious set at being Shakespeare whcih Shelley took for dramatic inspiration...."