The first line in the poem draws a line in the sand (or should we say the clouds?). The speaker establishes right from the beginning that she thinks the human brain has more scope than the sky. We wonder if this is a totally metaphorical comparison, or if she actually means to say that humanity as a whole is somehow superior to nature.
The Brain is deeper than the sea— (5)
This line is a direct echo of the first line. Again the speaker is saying that the brain is somehow cooler than a major natural force. Again, we wonder if this is only metaphorical, or if there's a subtext that humans are better than nature in some way. Do nature lovers have a right to be offended by this poem, or are they just being too literal if they are?
The Brain is just the weight of God— For—Heft them—Pound for Pound— And they will differ—if they do— As Syllable from Sound— (9-12)
The final quatrain compares the human brain to God himself, basically saying that we're on an even playing field with the big guy. When the speaker says that our brains and God might only differ "As Syllable from Sound" she seems to be claiming that the brain is shaped by intellect like a syllable, and God is a raw unshaped natural sound. What's interesting here is that this is the only quatrain where the speaker doesn't claim that the brain somehow exceeds a force of nature. Instead, she says they're pretty much equal.