Sometimes I wonder if the movies that permeated my consciousness as a child determined my later research interests. Take Groundhog Day. I watched it the other week with the kids and realized that it’s basically a late 20th-century version of Leibniz’s parable about Sextus Tarquinius. Both can be filed under the category: stories about assholes who are momentarily plucked out of the stream of existence so they may contemplate if they wish to renew their commitment to assholery. The fact that Phil Connors (Bill Murray’s character) chooses, eventually, to be good while Sextus defiantly refuses isn’t the only difference between the two narratives. In Leibniz’s tale the possible world that is actualized is one in which Sextus’s individual interests and the world’s fail to align: Sextus meets a grizzly end but his demise is also the catalyst for the founding of the Roman republic. In Groundhog Day the interests of the individual and the world line up: indeed, the learning curve the movie traces is Phil Connors’ gradual, very gradual discovery that dedicating himself to fulfilling the interests of other people is also the route, the only route, to his own happiness.
Before we watched the movie, I was sitting on the sofa and the younger sidled up to me, eyeing my belly critically.
“Are you going to have a baby, or what?”
I answered carefully. “No. No, I am not going to have a baby.”
Later that night, after the movie, when I was going to bed, I put on my Leibniz T-shirt, as you do (it has a portrait of Leibniz on it—I’ve been told it looks like me, truth be told, which is not a terribly flattering comparison—and underneath it reads, “In another world it would be worse”), and I wondered about that other world, the world the younger the imagined, in which she would be the middler.
Grammatically, when we express ideas about possibility, we use modal verbs like must, shall, will, etc.
This week, it being February, I had to teach “Frost at Midnight.” Duck-rabbit, I said sternly to myself, you shall. Not. Cry.
(I more or less pulled it off.)
“You shall not cry” is an example of something called “commissive modality” (I only learned this concept today.) It refers to a kind of modal statement in which the speaker indicates a strong commitment to ensuring the described event comes to pass (in this example, not crying). In English such statements usually use the modal verb “shall.”
There is something, I reflected today, especially poignant about second person commissive modality –i.e. statements in which shall is used in the second person, like “you shall not cry.” Indeed, it’s precisely when reading those lines when “Frost at Midnight”’s speaker uses shall (and it’s always in the second person, because he’s addressing his infant son) that my voice breaks:
“My babe so beautiful! it thrills my heart
With tender gladness, thus to look at thee,
And think that thou shalt learn far other lore,
And in far other scenes!
“But thou, my babe! shalt wander like a breeze”
“he shall mould
Thy spirit, and by giving make it ask”
“Therefore all seasons shall be sweet to thee”
“Or if the secret ministry of frost
Shall hang them up in silent icicles,
Quietly shining to the quiet Moon.”
Why is shall used in the second person poignant? I think it’s because of the other contexts in which we more usually encounter “shall,” which are quite specific.
Shall is the language of the law, whether divine and laid out in scripture (“Thou shalt not …” in the King James Bible or, to go back to Leibniz and his fable, “If you will renounce Rome, the Parcae shall spin for you different fates, you shall become wise, you shall be happy,” as Jupiter says to Sextus) or secular and expressed in legal documents, where it’s normally used in the third person (“the parties shall share joint legal custody”). The most prominent use of second person shall that I can think of other than in scripture is in fairy tales, where it’s used by fairies good and bad. “You shall go to the ball!” proclaims Cinderella’s fairy godmother. 
In the Ladybird book version of Cinderella I read as a child (the one with the three balls and, most importantly, the THREE DRESSES), in response to Cinderella expressing her wish to attend the ball the fairy godmother says, “And so you shall, my dear.”
Or consider these lines, uttered by the bad fairy in the Ladybird book version of Sleeping Beauty:
“When the King’s daughter is fifteen years old, she shall prick herself with a spindle and fall down dead.”
But then the good fairy modifies the bad fairy’s curse: “‘The Princess will prick herself with a spindle,’ went on the twelfth fairy, ‘but she shall not die. She will fall into a deep sleep that will last for a hundred years.’” 
When fairies or gods use shall in the second person, it’s not poignant because their supernatural authority underwrites their decrees ; the difference is when mere humans use “shall.” Such statements become poignant because of the auditor or reader’s understanding that the speaker can’t actually guarantee that the events they so confidently decree will necessarily come to pass. When an ordinary human uses “shall” in the second person, the very use of that verb bespeaks the individual’s powerlessness and their resultant desire to take on the authority of a god or fairy who might actually make the desired event so merely by uttering the decree.
Take Tennyson’s poem “The Death of the Old Year”: the first two stanzas end with second-person shall statements; “You came to us so readily, / You lived with us so steadily, / Old year you shall not die.” And then: “So long you have been with us, / Such joy as you have seen with us, / Old year, you shall not go.” But the point, as the poem makes clear, is that the speaker has no agency to stop the year from turning. The “shall” statements are a desperate bid to ward off the inevitability of death.
All of this is a very long way of explaining why “and therefore all seasons shall be sweet to thee” makes me cry; because would that all our benedictions for our babies were also promises.
 I looked but couldn’t figure out where this particular, now iconic expression of the fairy godmother’s promise to Cinderella derives from. Disney???
 I stumbled on an amazing subgenre of YouTube video while searching for these references: videos of actual Ladybird books being flipped through silently, sometimes with hands visibly turning the pages, other times not.