Throughout, the poem plays with language to shake up our ideas about who's sane and who's gotten hit by the crazy stick. It says "whatevs" to typical notions of sanity and boldly declares that people who might seem insane are actually the only ones with a clue, while so-called normal people are actually the ones who ought to be in straightjackets.
Line 1: The speaker doesn't beat around the bush here. She launches right into her craziness = sanity thesis right off the bat. It's kind of a paradox, right? At first it seems to contradict itself, but we can tell from the get-go that this speaker has a point to make. Like with any good paradox, this contradiction might just seem true by the time we get to the end of the poem. Also, notice how both "Madness" and "Sense" are capitalized, making us stand up and pay attention to the basic paradox of the poem. It's like the speaker is saying, "Here's this important word and this important word. Let's slam them together and see what we get."
Line 2: The symbol of the "eye" pops up pretty frequently when we're dealing with themes of insanity. Here, the speaker claims the only clear-sighted people are those who can see that insane people are actually the only ones around who know what the deal is. Everybody else needs to go get their eyes checked.
Line 3: The speaker doubles down on her paradox in the third line. Not only is she claiming that crazy people are sane, but also that sane people are crazy. Again, she capitalizes "Sense" and "Madness," emphasizing the switcheroo she does with the words. This wordplay isn't just for fun; it's a tight way to deconstruct our preconceived notions of what sanity is. We look at it from one angle and then another—and by the end we see it all in a totally new way.
Lines 5-6: The poem brings it all home with these two lines, when it claims that people who go along with the majority are the only ones who are blessed with the label of sanity, while anybody who dares to be different is immediately put on the list for electroshock. Ultimately, the poem is demanding that we look at the fact that sanity isn't objective. What's generally thought of as sane and insane is just something that society as a whole somehow agrees upon. And guess what? The majority can be completely and totally wrong.
Line 8: The poem ends by cracking us over the head with the symbol of a "Chain." Notice how the word is capitalized? Even more evidence that this isn't any old normal chain that's binding one supposedly crazy person; it's a big whopping symbolic Chain that represents the smothering oppression of the Majority over people who dare to say, "Wait? Am I crazy, or is everybody else insane?"