The Brain is just the weight of God— For—Heft them—Pound for Pound—
The speaker really pulls out all the stops with this final comparison, declaring that the brain is the same thing as the weight of God. Bold move, Emily.
Notice that this time she's saying that the brain is almost the same thing as something, rather than saying that it's deeper or wider or whatever.
She again echoes the structure of the first two stanzas by using the next line to get us ready to prove the point of her final comparison.
Don't miss how "Pound for Pound" also directly mirrors "Blue for Blue" from the second stanza.
And they will differ—if they do— As Syllable from Sound—
The speaker seems to doubt whether the brain is much different from the weight of God at all.
If they differ at all, though, she says they differ in the same way that syllable differs from sound.
Um, what does that mean exactly? This one's definitely up for interpretation, but here's one theory for you: Syllables are sounds that have been shaped by the human brain to be part of a word. Sounds, on the other hand, can be anything. They can be the raw, unshaped sounds of nature—waves on a beach, rain falling, wind in the leaves, etc.
So while the human brain and God have similar powers, the speaker seems to equate humankind more with structured thought and God with raw nature.
These sorts of thoughts probably would've ruffled the feathers of certain religious folk back in Emily Dickinson's day, but it's totally in line with the rebellious streak she showed toward the Christian God in a ton of her writing.
So what do you think, Shmoopers? Is the brain wider than the sky?