Much Madness is divinest Sense-

Here, I believeDickinson referred to the people considered crazy or mad were actually the ones who spoke the most truth.

To a discerning Eye-

Much Sense-the starkest Madness-

‘Tis the Majority 

She continues, saying how the majority of people saw the ones who thought outside the norm and who spoke sense as threats. The majority saw it as a disease to question what everyone else thought was sense and call it senseless.

In this, as All, prevail-

Assent-and you are sane-

Demur-you’re straightway dangerous-

And handled with a Chain-

Here, I believe she explains that to succeed you must agree to the norm and you won’t be ceen as a threat. Howvever, if you speak up or reject the ideas of what the majority already believes, you will be lock away.