There was a famous saying in my past life: Boys only think about one thing, and it’s gosh-darn disgusting.

  And I had to agree.

  Sets were disgusting.

  Oh, sure, they seemed innocent at first — just a collection of things, like a basket of apples or a pile of failed cultivation protagonists. But the moment you started looking closer, you realised sets didn’t obey normal human intuition. There were sets that contained themselves, sets that didn’t, sets that were infinitely larger than other infinities, sets that somehow refused to fit inside the sets they logically should. The moment you let your guard down, they hit you with Russell’s Paradox and left you questioning reality itself.

  And yet, despite my firm belief that sets were an affront to common sense, I found myself standing in front of a classroom full of students, about to teach them arithmetic — a subject built on the foundation of sets.

  “Master Jiang,” Zhang Xian piped up from his seat, arms folded in the infuriating way only a precocious twelve-year-old could manage. “If zero is nothing, how can it still exist?”

  I resisted the urge to sigh. Zhang Xian had that particular glint in his eye — the one that meant he was ready to make my life difficult for the next several minutes. I should never have encouraged critical thinking in these children. It was my greatest mistake, and I was now paying the price.

  I placed the chalk down and turned to face him, lacing my fingers together like a sage about to impart deep wisdom.

  “Zhang Xian,” I said, “imagine your mother tells you that if you keep pestering your instructor with impossible questions, you’ll get no dinner tonight.”

  Zhang Xian hesitated. “That won’t happen. My mother wouldn’t —”

  “This is hypothetical,” I said. “Now. If you have one dinner and you eat it, you have zero dinners left. Yes?”

  A nod.

  “But if your mother gives you no dinner to begin with, you still have a dinner — you have a dinner that is missing.”

  Zhang Xian frowned. “That doesn’t make sense.”

  “Exactly,” I said. “Zero is the existence of a missing dinner.”

  The class went silent.

  Zhao Qiang, who up until now had been carving something into his desk with the tip of his brush, squinted. “So zero is real?”

  “As real as your hunger will be if you question it too much,” I said. “Now, if we —”

  “But if zero is real, then shouldn’t we be able to divide by it?”

  I took a deep breath. “Zhang Xian, do you enjoy being my greatest headache?”

  He shrugged. “It’s my nature.”

  I rubbed my temples. The worst part was that I had actually been studying division by zero last night. Not in a serious, academic way, but in the vague, abstract manner that came when I was too awake to sleep but too tired to be productive. I had been thinking about limits — how division by smaller and smaller numbers approached infinity, how things like L’H?pital’s rule tried to make sense of indeterminate forms, and how I was slowly losing my mind trying to make these thoughts relevant to my situation.

  None of that, of course, was helpful in this exact moment.

  “If you divide something by zero,” I said carefully, “you are asking how many times nothing fits into something.”

  Zhang Xian nodded. “And the answer is…?”

  “An existential crisis.”

  The classroom was quiet again. I let it settle. Eventually, Zhang Xian let out a quiet, “Huh,” and leaned back in his seat.

  I took that as a victory and moved on.

  -x-x-x-

  It wasn’t until the students had gone out for their midday break that I let myself slump forward onto my desk, exhaling slowly.

  Somewhere outside, I could already hear Zhang Xian trying to convince Zhao Qiang that zero was a hoax invented by corrupt mathematicians.

  I let out a slow breath, rubbing my temples. Zhang Xian was going to be the death of me. I liked him — really, I did — but he seemed to be on a one man crusade to overturn my world view.

  It wasn’t just the questions — it was the sheer, relentless determination behind them. There was no hesitation, no moment of doubt, just an unwavering confidence that the entire world was built on flawed assumptions, and that he alone had been chosen to dismantle them.

  And worse, he was starting to infect the others.

  By the time I gathered the energy to step outside, I found Zhao Qiang standing with his arms crossed, brow furrowed in deep thought, while Chen Meili looked utterly exasperated. Some of the parents who lived or worked in Qinghe itself were there, visiting their children during the break.

  “It’s not that zero does or doesn’t exist,” she was saying, her patience hanging by a thread. “It’s that it represents an absence.”

  “But if it represents something,” Zhang Xian countered, “then it exists, doesn’t it?”

  Ru Lan, sitting a few paces away, tilted her head slightly. “Then does silence exist?”

  Zhang Xian paused.

  I had never been prouder.

  I turned just in time to see Ru Jingshan making his way toward me.

  Ru Lan’s father was a man of quiet strength, the kind of person who didn’t need to raise his voice to command attention. The first time we had spoken, it had been brief — polite greetings, a few words about Ru Lan’s studies, and a nod of approval before he had walked away. I hadn’t thought much of it at the time. But as I had come to learn more about the town, I had realized that Ru Jingshan was not just respected — he was known.

  He was the head of a family that, while not wealthy, was deeply entrenched in Qinghe’s affairs. His eldest son had already made a name for himself in the city, aiming for the imperial exams, while his other children had settled into respectable trades. And then there was Ru Lan, his youngest.

  I had assumed, at first, that she was quiet because she was the youngest in a house full of brothers. That she had simply learned to listen before speaking, to measure her words carefully. But the more I watched her, the more I realised it was something else entirely.

  She wasn’t just listening.

  She was watching.

  “Master Jiang,” Ru Jingshan said again, stopping before me. He gave a nod, his expression unreadable. “My daughter speaks well of your lessons.”

  I blinked. “She does?”

  “She says you teach them to think.”

  I resisted the urge to glance over at Zhang Xian, who had now begun a full-fledged debate with Ma Rui over whether infinity could be counted. How did a bunch of preteens even manage to navigate their way toward this topic?!

  “I try,” I said instead.

  Ru Jingshan regarded me for a long moment, then gave a slow nod. “That is good.”

  There was something about the way he said it that made me feel like I had passed some invisible test.

  And then, with nothing more than a polite nod, he turned and left.

  Ru Lan, still sitting nearby, watched him go. Then she turned back to me and said, “My older sister says Father speaks in judgments, not words.”

  I arched a brow. “And what judgment was that?”

  She considered this for a moment. “You’re interesting.”

  I wasn’t sure how I felt about that.

  -x-x-x-

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  How long had it been since I started teaching? A month? Two months?

  I wasn’t sure how I felt about a lot of things lately. My dantian was shattered, yet I was catching inkpots midair before they fell. My nights were supposed to be dedicated to mathematics, yet they were spent mentally defending myself against Zhang Xian’s increasingly convoluted attempts to disprove arithmetic.

  At least break time meant a temporary reprieve from existential crises.

  Or so I thought.

  Because as I turned back toward the schoolhouse, I saw Ma Rui and Zhao Qiang standing over a crudely drawn diagram in the dirt, sticks in hand, faces scrunched in deep concentration.

  That was never a good sign.

  I hesitated. Then, with the resignation of a man who had seen too much, I walked over. “What,” I said, “are you doing?”

  Ma Rui gestured at the ground. “We’re making a paradox.”

  I sighed. Why had I ever thought it a good idea to tell them about Zeno’s Paradox?

  “Why?”

  “So we can trap Zhang Xian in it.”

  I took a moment to process that.

  “You’re trying to trap him.”

  “Yes.”

  “In… a paradox.”

  “Yes.”

  Zhao Qiang nodded solemnly. “If we make a strong enough paradox, he’ll get stuck thinking about it forever.”

  That was, perhaps, the most diabolical thing I had ever heard.

  I looked down at the diagram. It was an absolute mess of circles, arrows, and what appeared to be a poorly drawn cow.

  “…Explain.”

  Ma Rui pointed at the center of the diagram. “Okay. So. This is Zhang Xian.”

  “Of course it is.”

  “And here,” he pointed at the cow, “is his logic.”

  I paused. “Why is it a cow?”

  “Because it’s going to get lost.”

  I blinked. “Go on.”

  Zhao Qiang took over, gesturing at the spiraling arrows. “First, we tell him that if he knows he’s in a paradox, then he’s not in a paradox.”

  I rubbed my temples. “Right.”

  “Then,” Ma Rui said, grinning, “we ask him if he can ever truly know he doesn’t know something.”

  I considered it.

  Then I considered the potential for peace and quiet.

  Then I said, “Carry on.”

  And walked away.

  -x-x-x-

  By the time the students returned to the classroom, I had almost managed to convince myself that the rest of the day would be normal.

  That was a mistake.

  “Master Jiang,” Chen Meili said as she settled into her seat, “is it true that numbers are made up?”

  I frowned. “What?”

  “I heard Zhang Xian say that numbers are fake and only exist because we believe in them.”

  I turned slowly to Zhang Xian.

  He shrugged. “What? If you take away all the numbers, where are they? Nowhere. That means they were never real to begin with.”

  I took a deep breath.

  Then I turned to the board and, in large, deliberate strokes, wrote:

  “2 + 2 = 4”

  And underlined it.

  “This,” I said, “is real.”

  “But—”

  “No. No buts. This is a fact. If I give you two apples, and then two more apples, you will have four apples.”

  “Unless you take one away,” Ma Rui muttered.

  “Or if one of them is actually a pear,” Zhao Qiang added.

  I pinched the bridge of my nose. “We are not going to deconstruct numbers today. We are going to —”

  “Master Jiang,” Ru Lan interrupted, voice as soft and steady as ever, “what if there’s a number that exists but we can’t see it?”

  Silence.

  Slowly, I turned to her.

  Because that — that was an actual, real question. A good one.

  She met my gaze calmly.

  I exhaled. “Like… an imaginary number?”

  Ru Lan tilted her head. “Is that a real thing?”

  “Yes,” I said, feeling some combination of exhausted and delighted. “It is.”

  I turned back to the board and, beneath the underlined 2 + 2 = 4, I wrote:

  i = √-1

  There was a long pause.

  Then Ma Rui squinted. “That’s illegal.”

  “No, it’s imaginary.”

  “That’s the same thing.”

  “It is not.”

  Zhang Xian leaned forward, suddenly intrigued. “So you’re saying there are numbers that don’t exist, but they do?”

  “Yes.”

  “But you said numbers are real.”

  “Yes.”

  “So how —?”

  “They exist in a different way,” I said quickly. “Just like zero. You can’t see it, but it has meaning.”

  What were they putting in xianxia water? What made children even ask questions like these?

  Then again, this was a world where children started learning how to smash boulders with their bare fists before they even turned twelve.

  Zhang Xian stared at the board, gears turning in his head.

  And then he smiled.

  I felt a chill run down my spine.

  “I have so many questions,” he said.

  I regretted everything.

  -x-x-x-

  By the time the day ended, I had fielded no fewer than eight questions about whether numbers were secretly sentient, had to ban Zhang Xian from writing a proof that 1 = 0, and had listened to Ma Rui propose a theory in which all math was a vast conspiracy by the imperial court to control grain taxation.

  By the time the last student had left and I was alone in the classroom, I felt like I had aged at least ten years.

  I slumped into my chair, staring at the chalkboard where i = √-1 still sat, underlined, as if daring me to reconsider my life choices. Outside, the town moved on as if nothing had happened — merchants selling their wares, farmers pulling carts of produce, blacksmiths hammering away at metal. A world blissfully unaware that I had spent the last few hours wrestling with the existential consequences of whether numbers were a government conspiracy.

  I exhaled, rubbing my temples.

  What was I even doing?

  When I had first taken this job, I thought it would be simple. Teach some arithmetic, drill in some memorized passages, collect my pay to tide me over while I did my independent studies in mathematics. But now?

  Now my students were attempting to trap each other in paradoxes, questioning the fabric of reality, and potentially inventing entirely new branches of philosophy by accident.

  At this rate, I was going to have an entire classroom of heretics.

  And the worst part was — I wasn’t sure I could stop them.

  Because it was now clear that something was happening. Something beyond just bright students and childhood curiosity.

  I had been in the classroom long enough now to notice patterns. And the pattern was this: none of them were arguing like this in the other subjects I taught them. They memorised their classics, copied their passages, read their histories. They discussed the interpretation of proverbs when I bid them to, but they didn’t engage in hour-long debates about their meaning during their midday break the way they did the notion of whether ‘0’ was a natural number. Instructor Liang still ruled his older students with an iron glare, and Miss Shen’s young class still ran on innocent giggles and ink-stained fingers.

  But in my classroom? The students debated the nature of numbers, tried to prove paradoxes, and were rapidly approaching the dangerous precipice of mathematical philosophy.

  At first, I thought it was just Zhang Xian being a menace, infecting the others with his constant questioning. But then I looked at Ru Lan.

  She had been my quietest student at first. The one who had cried in my very first lesson, frozen by numbers she couldn’t grasp. But now?

  Now she was asking me about imaginary numbers.

  Now she was arguing the existence of things unseen.

  Now she was breezing through my lessons like they were easy.

  I didn’t know how to feel about that.

  I had been excited at first — of course I had! What teacher wouldn’t be proud of a student improving? But this wasn’t just improvement. It was unnatural. And it wasn’t just Ru Lan. It was all of them, every single one of my students, and only in mathematics.

  They weren’t just learning fast. They were learning in ways that children their age shouldn’t.

  They were making logical leaps, jumping between concepts that should have taken years of careful instruction. They weren’t following the standard, gradual progress of arithmetic into algebra into geometry. No — they were discovering mathematical ideas entirely on their own, chasing patterns, constructing arguments, making connections.

  And I —

  I was afraid.

  Because I had no idea why it was happening.

  My dantian was shattered. My qi should have been — was — nonexistent. I had abandoned the path of cultivation entirely. But somehow, I was affecting them.

  No. Not affecting them. Influencing them.

  And that was a deeply unsettling realization. They were only children.

  Because I didn’t understand how.

  And if I didn’t understand how, I couldn’t control it.

  -x-x-x-

  I needed to test this. I needed to know if it was truly me or if I was just being paranoid. So, I did what any responsible adult would do.

  I decided to gaslight a classroom full of children.

  The next morning, I walked into class, set down my chalk, and announced, “We are doing multiplication drills today.”

  A collective groan rippled through the room.

  “But Master Jiang —”

  “No ‘but Master Jiang’s.’ We’re reinforcing basic skills today. No debates, no paradoxes, no conspiracies.”

  Zhang Xian crossed his arms, scowling. “This is an oppressive system of control.”

  “Yes, it is. Thirteen times seventeen?”

  The next hour passed in excruciating boredom. I kept the lesson deliberately mechanical — rote memorization, repetition, drills. The energy in the room died. Even Zhang Xian, the eternal agitator, was subdued, droning out multiplication tables like a prisoner scratching tally marks on a wall.

  And I watched.

  I watched for any sparks of curiosity, for any sudden bursts of brilliance.

  There were none.

  For one full hour, they were normal children.

  And then, when I finally relented and introduced a single logic puzzle — just a harmless little number trick — everything changed.

  It was like striking a flint. The moment I posed the question, their eyes lit up. Zhang Xian immediately tried to deconstruct the problem into its core assumptions, while Zhao Qiang grabbed a scrap of parchment and started scribbling. Ru Lan tilted her head, brow furrowed, already thinking through the implications.

  The moment I engaged with mathematics beyond simple memorisation and computation, it was like the entire class had shifted gears.

  And that was when I knew.

  It was me.

  It wasn’t just Zhang Xian. It wasn’t just a bright class. It wasn’t even some strange genius of Ru Lan’s.

  It was something about me, about how I taught, about whatever unexplainable thing had happened to my cultivation despite shattering my dantian.

  Because, yes, that was it. I could see no other possibility. An unproven conjecture, yes — but the Riemann Hypothesis was just as unproven, and it had been checked to the first 1013 zeroes. Hundreds of theorems exist bearing statements that begin under the assumption that the Riemann Hypothesis was true.

  Somehow, I was still cultivating.

  -x-x-x-

  That evening, I sat at my desk, staring at a blank page.

  What did I even do? How did I fix this? How did I contain this?

  I had believed I was powerless. That I was the same regular mortal I had been back on Earth. That my dantian was gone, that my connection to cultivation had been severed. And yet, here I was, warping reality and young minds through mathematics.

  This was a problem.

  This was a moral problem.

  What if this spiraled out of control? What if I wasn’t just influencing their thoughts, but their minds? What if this was something more than just quick learning — what if I was changing them?

  I couldn’t afford to be careless. I had to control this.

  I had to stop them from spiraling further, from reaching concepts too early, from asking questions they weren’t ready for.

  And above all, I had to make sure I wasn’t doing harm.

  I reached for a fresh sheet of parchment and, on impulse, wrote down one of the first mathematical thoughts that had truly terrified me:

  “There exist infinities larger than other infinities.”

  I stared at the words, heart pounding. I almost crumpled the paper on the spot.

  This was a xianxia world. Where people unprepared for grand truths had their cultivation shattered. Where a contradiction in one’s world view could lead to their Dao hearts fracturing, all the way through to their heads exploding.

  My students were proving remarkably intelligent, but I wasn’t certain that all of them were ready to comprehend Cantor’s theory of infinite sets. And yet, they were dangerously close to approaching that topic.

  I burned that sheet of parchment in the candle flame.

  I had to control this.

  Before one of them inevitably disproved themselves.