"Things fall apart; the centre cannot hold"
This little quote has been referenced countless times in other works since this poem was written; it is the title of Chinua Achebe's critically acclaimed novel "Things Fall Apart" and has inspired many other works such as David Gulden’s photography collection The Centre Cannot Hold, as well as the book American Empire: The Center Cannot Hold by Harry Turtledove.
On an essential level, the first part means what it says: things fall apart. Things go wrong, relationships end, systems of power collapse—often destroyed from the inside out.
"The center cannot hold" essentially means that the center or core of meaning, logic, and reason is collapsing, coming apart.
There is a theory of the universe that states that the universe is constantly expanding, implying that the universe is constantly in a state of "coming apart," spreading away from its center. But if "the center" itself "cannot hold," what does this mean for the future of the world? Where does the center go? Perhaps it leaches out to the outskirts of the world, lending to the universe's nature to operate in cycles. Perhaps there is a black hole forming in the center of the universe, a portal to the beyond, a portal to some other dimension.
Regardless, this sentence has cosmological implications and seems to talk about the fate of the universe on a vast scope, and makes it clear that something is changing irreversibly.
"The best lack all conviction, while the worst are full of passionate intensity"
This quote implies that the best of people, perhaps the good or moral, seem to have no determination and no ability to stand up for themselves—while the most evil are the most dedicated and loud about their beliefs. It mourns the failure of people who believe in goodness, kindness and compassion to triumph over chaos and violence.
"The darkness drops again; but now I know that twenty centuries of stony sleep were vexed to nightmare by a rocking cradle"
These "twenty centuries of stony sleep" likely refer to the twenty centuries, or nearly 2,000 years, since Christ's birth and the beginning of Christianity. By referring to the reign of Christianity as "stony sleep," Yeats implies that Christianity was glossing over or covering up some primal energy that reigned before its emergence and that this primal energy is soon going to return.
These "twenty centuries of sleep" were "vexed to nightmare," or brought to the destructive, surreal scene that Yeats describes at the end of the poem, by a "rocking cradle." This "cradle" is most likely human civilization itself, which, like the poem's "falcon," is helplessly at the mercy of greater forces, and which is losing contact with its falconer, its God, or its sense of order. The "rocking" is likely the violence that has shattered humanity, disturbing its calm sleep in the arms of faith and tradition and creating a nightmare—or a delirious kind of freedom—that emerges in the form of the "beast" of the future.
"And what rough beast, its hour come round at last, slouches towards Bethlehem to be born?"
This line asks the question we've probably all been asking: what will this apocalyptic Second Coming actually look like? By asking "what rough beast," Yeats is saying that he knows that the Second Coming can be represented by the shadow of a beastly creature, but he does not know exactly what the creature will be. He does know that it has been biding its time, waiting to emerge. It is in no rush to arrive.
The last phrase, which provided the title for Joan Didion's book of essays Slouching Towards Bethlehem and which, along with the rest of the poem, inspired Joni Mitchell's song "Slouching Towards Bethlehem," is noteworthy for a few reasons. First, it uses the word slouches. Yeats could have picked any number of words to describe the beast's coming, but he picked slouches—a word that implies, if not a kind of laziness, then a kind of languorousness, a slow, ominous plod. The beast is not charging, for it knows it does not have to put up a fight—humans have already paved the way for its emergence, having created the perfect alchemy of chaos for its birth.
The last word, Bethlehem, sews the poem neatly into a cradle of Biblical references gone wrong. Yeats's title, "The Second Coming," refers to the Book of Revelations, and Bethlehem is the city in which Jesus was born to Mary. This new beast will be a sort of Christ, emerging from Bethlehem—but it will not be Jesus. It will be something very unorthodox. It will be something else entirely.
