The poem's opening stanza sets up its argument: the speaker declares that it's not necessary to be a room or a house to be "Haunted." People would typically link haunting to a physical space filled with lurking ghosts, but the speaker argues that hauntings can, in fact, happen right within the mind.
To support this point, the speaker presents the mind as a mysterious and complex place that, like a house, has "Corridors" (or passageways). In fact, the speaker finds the passageways of the mind to be even longer, darker, and more mysterious than any real, physical corridor (than any "Material Place")! This description suggests that it's easy to get lost in the mind.
Dickinson uses various poetic devices throughout these lines in order to emphasize the poem's point. Lines 1 and 2, for example, feature anaphora—repeating the phrase "One need not be a" to hammer home the idea that it's not necessary to be a physical house or room to be plagued by ghosts.
These lines also feature clear sibilance, as in:
The Brain has Corridors — surpassing
Material Place —
Note how those hissing /s/ sounds add a sinister hush to the speaker's description of the mind.
Finally, this opening quatrain establishes the poem's iambic rhythm. An iamb is a poetic foot with an unstressed-stressed syllable pattern. As readers can see here, the first and third lines of each stanza are much longer than the second and fourth:
One need | not be | a Cham- | ber — to | be Haunted —
One need | not be | a House —
The Brain | has Cor- | ridors | — surpassing
Mater- | ial Place —
Dickinson often uses something called common measure in her poetry: lines of iambic tetrameter (four iambs per line) alternating with lines of iambic trimeter (three iambs per line). However, the meter in this poem isn't steady:
- line 1 has five iambs (making it iambic pentameter) plus a dangling unstressed beat;
- line 2 as three iambs (making it iambic trimeter);
- line 4 has four iambs (iambic tetrameter) with another dangling unstressed beat;
- and line 4 is actually an iamb followed by an anapest (a poetic foot that goes unstressed-unstressed-stressed).
It's like the poem is nodding toward a common measure but stumbling, as if the meter is itself lost and disoriented.