What is the message and the themes in Dickinson's poem "One need not be a chamber"?

The poem "One need not be a chamber to be haunted" deals
with a contrast between fears caused by external phenomenon and those caused by our own
brain.  We see this contrast in the first stanza when the speaker maintains
that

The brain
has corridors surpassing

Material
place.

In other words, what
is inside our brain is much more frightening than any tangible thing or place.  This
idea is further developed in the second stanza in with the "interior" is more dangerous
than an "external ghost."  The speaker declares that we can arm ourselves against the
"assassin, hid in our apartment," but cannot defend ourselves against the intruders of
our minds.

Dickinson is vague as to what we have within
that is so frightening.  But it could be any number of things:  regret, guilt,
imagination, anxiety, paranoia, fears of loss or death--anything that makes us fearful
to continue on with life or reluctant to get out of bed.