The poem uses parallelism (and the more specific device anaphora) in each of its three stanzas, repeating the same grammatical structure again and again and beginning many of its lines with the same words and phrases. This creates a sense of building logic in the poem even as the speaker compares the human mind to increasingly remarkable things.
The speaker begins each stanza with the exact same phrase: "The Brain is." This anaphora is then followed by the speaker's comparing the “Brain” (that is, the human mind) to the sky, the sea, and then to God. She then asks the reader to imagine holding the “Brain” up next to each of these things. Finally, she concludes each stanza by proving her claim, explaining why the “Brain” is “wider than the Sky,” “deeper than the sea,” and “just the weight of God.”
Note how similar the first three lines of stanzas 1 and 2 are in particular:
The Brain is [wider/deeper] than the [Sky/sea]
For [put/hold] them [side/Blue] [by/to] [side/Blue]
The one the other will [contain/absorb]
Only a few words change here, and the gist is generally the same. This repetition, in turn, emphasizes the mind’s limitless capacity to perceive the surrounding world.
Interestingly, the poem’s parallelism also calls attention to the moments when the speaker shifts out of this structure. Specifically, in stanza 3, the speaker doesn’t claim that the “Brain” is greater (“wider” or “deeper”) than God. Instead, the speaker says that the mind is “just the weight” of God, meaning that the human mind is similar to God, but not more infinite.
The speaker also doesn’t say that the human mind can “contain” or “absorb” God, abilities emphasized in the poem’s beginning. Instead, the speaker introduces an element of doubt, suggesting that the mind might actually differ from God in some way.