Emily Dickinson's poem expresses the paradox that what is sometimes regarded as madness or craziness is actually perfectly sensible, and what sometimes seems to make perfect sense is actually madness. The paradoxes are reversals of each other. These paradoxes are the statement of the theme of the poem. The poem goes on to explain her a point a bit further. She explains that it is the majority (of people) who decide what is "sane" and if one goes along with the majority, then that he/she is considered sane, but that if one demurs (objects) then he/she is considered crazy and will be treated that way.
While we have no expressed idea about what inspired this poem for Dickinson, we can certainly see its relevance in all kinds of big and small examples in our everyday lives. If the majority of people decide that a certain fashion trend is great, then anyone wearing that trend is considered fashionable; anyone who doesn't, isn't. This could apply to anything that is "popular." Dickinson's reclusive behavior might have been considered mad, but it made perfect sense to her and how she wanted to live her life. Dickinson's first and only submission to have her poetry considered for publication ended in a rejection letter, but she knew she wrote great poetry and we certainly know that today! Who is to say what is mad and what is sane?
In this poem, Dickinson presents two paradoxes, or statements that contradict themselves. The first is that people with "much madness," or who are insane, show a great deal of sense. In fact, people who act insane show the type of sense that characterizes the divine. The second paradox is that those who show "much sense," or who seem totally sane, in actuality show "stark madness," or complete insanity.
The poem's message is that those who "assent," or give into society's rules and norms, are considered sane. On the other hand, those who "demur," or who are reluctant to follow the dictates of society, are considered dangerous and "handled with a chain," or treated with barbarism. In other words, the label of sanity is awarded to those who simply follow the rules, and those who disagree with the rules, even if the rules are themselves insane, are considered mad and handled with brutality. Dickinson expresses the idea that society's rules can be insane and that one who resists these rules is in fact closer to godliness and sanity.