34: Chapter 34 Folding Forty Years into a Single Page

When he was young, Jiang Lin liked to create a separate directory for every course, every stage, and every problem.

[Ordinary Differential Equations _ Second Round]

[Mathematical Methods in Physics _ Boundary Condition Index]

[Partial Differential Equations _ Heat Conduction and Wave Equations]

[Complex Variables _ Residue Method and Integration Techniques]

[Numerical Methods _ Finite Difference Experiments]

[Electrodynamics _ Fields and Boundaries]

[Thermodynamics and Statistical Physics _ Entropy and Free Energy]

[Modern Physics Experiments _ Low-Credibility Replication Records]

Back then, folders were like flags.

Every time he created a new one, it was like driving a stake into a new continent, telling his future self:

"I have been here, I am conquering here, and this place will be sorted out by me sooner or later."

After the twentieth year, Jiang Lin slowly stopped doing this.

It was not because there was less to learn.

On the contrary, the further he went, the more knowledge resembled a cluster of ruins deep in the Wasteland, one connected to another, with no end in sight at all.

For some courses, he was not qualified to say he had truly conquered them, only that he knew the entrance.

For some tools, instead of leaving sudden realizations in the time capsule, they had quietly seeped into his hands.

They became boundary conditions checked subconsciously during the next modeling, outliers casually deleted during the next fitting, and dimensions that he dared not omit during the next derivation.

He gradually came to know that it is impossible for a person's life to name everything into chapters.

Archiving itself requires energy.

And after the twentieth year, Jiang Lin began to precisely conserve his attention.

This thing was harder to replenish than Soybeans and potatoes.

In the twenty-first year, a relatively noticeable double vision appeared in his right eye for the first time.

That day, he was reading a lecture note on mathematical methods in Physics.

On the screen was a string of boundary conditions and eigenfunction expansions.

Suddenly, he felt there was a shallow phantom layer beside the characters, like a fainter shadow standing next to each symbol.

He closed his eyes, rubbed them, and opened them again.

It was still there.

In the twenty-second year, Jiang Lin put on reading glasses, and an area at the lower right of the electronic ink screen stopped refreshing.

At first, it was only the size of a fingernail, but later it expanded to half a palm, like a gray scar permanently remaining on the screen, blocking a few lines of annotations on the page margin.

Jiang Lin tried to fix it.

Dismantle the casing, clean the ribbon cables, remove dust, reboot.

Cold start, change the backup cable.

It was useless.

The wound on the screen was right there.

He did not take it apart a second time.

Dismantling it again might completely ruin the entire screen.

So he changed the layout.

He increased the right margin of frequently used documents, moved all annotations uniformly to the left, and changed the original double-column reading to single-column.

In the twenty-third year, he began to write reviews using shorter sentences.

The reviews of his early years were like classroom notes.

Written out paragraph by paragraph, wishing he could leave every detail of today's thinking to the future.

Not anymore.

He still had plenty of time.

But he became increasingly clear that having a lot of time did not mean having a lot of effective energy.

The time periods in a day when he was truly awake, stable, and able to handle difficult problems were limited.

These time periods could not all be spent writing pretty reviews.

Therefore, his reviews began to turn into extremely short judgments.

[Boundary conditions are more important than equations.]

[Ask about the scope of application for models first.]

[Low-credibility data does not enter the master table.]

[Numerical results do not explain physical images.]

[If you don't understand, you don't understand; don't use metaphors to save face.]

The last sentence was written for himself to see.

When he was young, he loved metaphors very much.

These metaphors helped him survive many of the most difficult thresholds.

But later on, he also began to be wary of metaphors.

Because a metaphor is only a bridge, not the other shore.

A bridge can save lives.

But one cannot mistake the bridge for the opposite bank.

In the twenty-fourth year, the wind-powered water-lifting device, renovated year after year, was finally retired.

The water level in the underground rock fissures had changed a few times. Later, he modified the water storage system, made more stable water collection troughs in several low-lying areas, and optimized the treatment process for acid rain during the rainy season once more.

The wind-powered water-lifting device could still be used, but the maintenance cost was getting higher and higher.

So he took it apart.

Everything that could be recycled was recycled, and the remaining cracked fulcrum stones were moved by him to the edge of the field ridges to press down grass.

In the twenty-sixth year, the capacity of the third storage battery dropped to the danger line.

Jiang Lin had long had a contingency plan.

The low-power mode had been running for many years, and the use of the backup computer was suppressed to a minimum.

The paper indices were enough to maintain his learning skeleton under extreme power outages.

But when that battery truly retired, certain learning methods also completely ended.

Watching videos for a long time, ended.

Repeatedly opening large PDFs for comparison, ended.

Temporarily calling up massive amounts of data for horizontal retrieval, ended.

In the future, he would have to rely more on the index in his own brain, minimalist notes, and internalized knowledge frameworks.

Over the years, hard drives and batteries had given him a nearly luxurious starting period.

Now, the equipment began to age one by one.

Like teachers, they taught him things and then exited the stage.

In the twenty-eighth year, a small-scale degradation appeared in the farmland.

The yield of several potato ridges on the southern side declined for two consecutive seasons, the leaf color was somewhat pale, and soil tests showed that neither pH nor organic matter was bad.

Jiang Lin initially suspected the degradation of seed potatoes.

He changed a batch of reserved seeds.

The improvement was limited.

He suspected moisture.

He adjusted the watering.

Still not obvious.

Later, he dug out years of records and found that those ridges had borne too many high-yield crops over the past five years, straw returning to the field was insufficient, the deep-layer structure had worsened, the surface data looked okay, but the downward space for the root system had been slowly crushed.

This was not a single-variable problem.

Nor was it caused in a single year.

It was like a person.

Barely holding on on the surface, while deeply exhausted for a long time.

When writing this sentence, Jiang Lin touched his own knees.

In recent years, crouching down and standing up again on cold days always took half a beat slower.

It was as if a bit of fine sand was hidden in his joints, grinding very lightly, yet grinding for a very long time.

In the thirtieth year, Jiang Lin spent the forty-eighth birthday of this lifetime in the Wasteland.

Of course, there was no cake.

Nor would there be candles.

There wasn't even a birthday celebration in the accurate sense.

He only converted according to the date and knew that this day had arrived.

In the morning, he checked the water as usual; in the morning, he organized an old notebook; in the afternoon, he replenished compost for the farmland.

In the evening, he boiled a little more Soybeans.

His teeth were already a bit poor, so the Soybeans were boiled softer than usual.

He chewed slowly and swallowed carefully.

Because his gums couldn't take hard things, and his stomach no longer liked swallowing too hastily.

When he was young, eating food was replenishing fuel.

Now eating food was more like maintaining an old machine: it couldn't be too hard, couldn't be too cold, and couldn't be too fast.

Outside the house, the wind was not loud, and the stove firelight was steady.

Spread out on the table was the twenty-fifth volume of his concept notebook.

He suddenly thought that as long as he went back, his body would refresh.

But Jiang Lin in the Wasteland was already forty-eight years old.

The sense of tearing between the two would only be relatively strong when returning.

At this moment, he would not obsess over whether he should be considered eighteen or forty-eight.

Physical age is one state, and memory age is another state.

Social identity is yet a third state.

There was no need to forcibly compress them into a single number.

Linear Algebra had long taught him that a system can have multiple components.

He was simply a state vector.

After the thirty-fifth year, Jiang Lin stopped opening new courses on a large scale.

This decision was not made suddenly on a certain day.

One early morning, while organizing the course directory, he found himself dragging the same folder into different classifications three times in a row.

Another night, looking at a proof where he clearly recognized every single word, he could not recall how he had already written about a similar sticking point in the nineteenth volume of his concept notebook three years ago.

Further down was his waist.

After sitting for a long time, a dull pain would well up deep in his lower back, like old wood slowly bending after getting damp.

He had to support himself against the stone table for a while, wait for the dull pain to recede, and then sit back down.

He could still continue.

But the act of continuing itself was no longer as cheap as it was when he was young.

He had grown old in the Wasteland.

In the spring of the thirty-eighth year, Jiang Lin took the first volume of his concept notebook out of the moisture-proof box.

Opening the first page, he saw his eighteen-year-old self write down that forcefully written sentence.

[The limit is not about looking at whether the front is messy, but whether the back can remain stable.]

Next to it was drawn a potato yield chart.

Crude, naive.

Statistical significance was a complete mess.

Yet the fifty-six-year-old Jiang Lin only wrote four small characters next to it.

[Direction is right.]

After that day, he truly began to enter the final stage of the fourth Wasteland.

No longer charging forward.

But looking back.

Sorting out the drafts, concept notebooks, electronic files, experimental records, course indices, and Wasteland engineering archives left over from the past thirty-plus years, one by one.

He gave this stage a very earthy name.

[Recycling Project]

It was not a summary, nor was it a compilation of achievements.

Summary sounded too decent, and a compilation of achievements looked too much like something meant for others to see.

Recycling was more accurate.

Over these years, he had learned too many things, and too many things had scattered away.

Some knowledge had already been internalized into intuition, some remained stuck on the edge of half-understanding, some felt important back then but later proved to be just a detour, and some looked inconspicuous, yet years later became the skeleton that could truly support him.

What he needed to recover was memory, understanding, thinking pathways that had turned into bodily reflexes, and topological information within the nervous system.

Therefore, in the final time, what he needed to do was not continue stacking knowledge.

But rather compress the things stacked over nearly forty years into structures that could be re-invoked by an eighteen-year-old body as much as possible.

In the thirty-ninth year, Jiang Lin made his first calibration table.

He wrote at the top of the table:

[Fourth Wasteland Knowledge Skeleton Calibration]

Column 1: Already Conquered.

Column 2: Can Be Rapidly Rebuilt.

Column 3: Requires Real World Calibration.

Column 4: Cannot Fake Understanding.

Then, he began to fill it out item by item.

[Advanced Mathematics]

Already conquered.

Not the kind of knowing how to differentiate or integrate.

But limits, continuity, derivatives, integrals, series, multivariate functions, and vector analysis had become the basic language with which he viewed the world.

When he first started learning, the ε-δ definition of limits confused him for a very long time.

He remembered writing a line in his concept notebook, the general meaning of which was: "Why go to such trouble, isn't it quite obvious intuitively?"

Later he learned that intuition doesn't count, proof does.

This was the first thing mathematical analysis taught him, and also the most important thing.

More than thirty years later, calculus was no longer a tool in his hands, but his eyes.

When looking at a cooling curve, he would instinctively think of functions.

When looking at the daily output of solar panels, he would instinctively think of integrals.

When looking at farmland yield fluctuations, he would instinctively think of limits, errors, and convergence.

When looking at the wind-powered water-lifting device, he would instinctively think of variables, constraints, and rates of change.

It was not about knowing how to solve problems, but that the way of viewing the world had changed.

But more advanced theories such as strict real analysis, measure theory, and functional analysis were not included.

He had read Lebesgue integration and could recount the definition, but had never been forced by any practical problem to actually use it.

This block was most likely empty and he could not fake understanding.

[Knows the entrance, needs Real World re-studying.]

[General Physics]

Already conquered.

Mechanics, thermodynamics, electromagnetism, vibrations and waves, and optics were no longer a collection of question types, but things he had experienced personally over and over again in the Wasteland.

Mechanics was not a cart on an inclined plane.

It was which brace would fail first when the solar panel support shook in the wind.

Thermodynamics was not the ideal gas law.

It was how the temperature curve approached the Wasteland night temperature after the fire in the Stone House went out.

Electromagnetism was not just a few formulas for electric and magnetic fields.

It was sunlight falling onto the panel surface, charges separating, current entering the storage battery, and then turning into a line of text on the computer screen.

Vibration was not a spring and ball.

General Physics no longer needed to be re-studied.

What needed to be re-studied were standard experiments, precision measurements, and the university experimental system in the Real World.

His Physics intuition was very solid, but his experimental conditions were too terrible.

This point had to be written clearly.

[Solid physical image.]

[Insufficient experimental training.]

[After returning to reality, standard experiments must be made up.]

[Linear Algebra]

Already conquered the lower-division core.

Vectors, matrices, systems of linear equations, rank, linear independence, eigenvalues, and orthogonal projections were no longer just problems.

He would write the Stone House as a state vector, view the farmland as state iteration, and view the solar system as multi-variable coupling.

He viewed the long-term changes of the Wasteland ecology as a rough matrix evolution.

He knew that having many variables did not mean having a lot of information.

He knew that rank was not a number game, but effectively independent directions.

Some variables looked different, but in reality they were just different facets of the same thing. After removing redundancy, the directions that could truly prop up the system were far fewer than they appeared.

He knew that characteristic directions were not cold words in textbooks, but the directions where a system was most prone to stabilizing, most easily amplifying, or most easily collapsing from.

This set of understanding was forced out by Quantum Mechanics.

Operators and commutation relations pushed Linear Algebra into a deeper place. He hit a wall in Quantum Mechanics and then came back to re-understand the composition of linear maps, only to truly understand what non-commuting matrices meant.

The order was reversed, but the result was solid.

He now understood the geometric meaning of determinants.

He understood the relationship between quadratic forms and positive-definite matrices.

There was one place he was not sure about: Jordan canonical form.

He knew the theorem and could recite it, but had never encountered a problem that required actually using it.

Next time he would find a few problems to do and verify it.

Undergraduate Linear Algebra was no longer an obstacle for him.

But abstract algebra, functional spaces, and infinite-dimensional operator theory were not included; he could not fake understanding.

[Probability, Statistics, and Error Analysis]

Undergraduate foundations already conquered.

Mean, variance, standard deviation, covariance, correlation, least squares, linear regression, and error propagation had already entered his daily judgments.

He would no longer treat feeling effective as actually effective.

He would not treat a sunny day's solar output as a system optimization result.

He would not directly attribute a certain year's potato yield increase to a single variable.

He would not mix low-credibility data into the master table.

He knew data could be dirty, errors could deceive, and correlation was not causation.

He knew a pretty fit might just be the Wasteland mocking him.

But modern statistical inference, experimental design, Bayesian methods, high-dimensional statistics, and machine learning theories still needed to be re-studied in reality.

Being able to use them did not mean being able to do research.

[Basic statistical thinking has been internalized.]

[Modern statistical system needs re-studying.]

[Ordinary Differential Equations]

Undergraduate foundations already conquered.

First-order equations, second-order linear equations, vibration equations, damping, forced vibration, stability, phase portraits, and linearization could all be rebuilt.

This was not because of how many problem sets he ground through.

But because the Wasteland was full of systems evolving over time.

He had long ceased to fear a certain quantity changing over time.

If a problem lacked a time term, he would instead suspect the model was too crude.

But at the theoretical level, he did not pursue it deeply. He knew the conclusion of the existence and uniqueness theorem of solutions, but could not prove it himself. For the deep theories of nonlinear systems, chaos, and strict dynamical systems theory, he could only write:

[Knows the entrance.]

[Cannot fake understanding.]

[Partial Differential Equations and Mathematical Methods in Physics]

The tools needed for undergraduate Physics could already be used.

Heat conduction equations, wave equations, Poisson equations, Laplace equations, Fourier expansions, orthogonal functions, separation of variables, and Green's functions—he wouldn't dare call himself proficient in these things, but he was no longer frightened by them.

Fourier series and Fourier transforms were the most solid parts among them.

The conversion between the momentum representation and position representation in Quantum Mechanics pushed the Fourier transform to a deeper position, knowing it was not just a tool for decomposing signals, but a translation between two descriptive languages.

But he knew that the equation itself was not everything.

Boundary conditions were the hand that reality reached into mathematics.

For the same equation, different boundaries meant different worlds.

He had repeatedly seen the weight of the word "boundary" in such problems as the insulation of the Stone House, the light absorption of the solar panels, the diffusion of moisture in the farmland, and the sound of the wind passing through the door cracks.

Special Functions (Legendre polynomials, spherical harmonics): He could use them, but he was unsure of his level of understanding.

The angular momentum part of Quantum Mechanics used these things all over again; he got the questions right, but sometimes it felt like he was just going through the motions rather than truly understanding.

This difference was one of the most important things he had learned to distinguish over the years.

Going through the motions left nothing behind except the steps themselves, which was even more dangerous than pseudo-understanding, because pseudo-understanding at least had a wrong comprehension propping it up.

[Undergraduate Physics and Mathematics tools available.]

[Strong awareness of boundary conditions.]

[Inadequate rigorous mathematical theory; cannot pretend to be a mathematics major.]

[Functions of a Complex Variable]

Instrumental mastery.

Residue theorem, contour integration, analytic functions, and series expansion—these were things he could use to serve Physics problems.

He knew the complex plane was not just for show.

Some paths that were difficult to walk on the real axis could actually be traversed by making a detour into the complex plane.

But as for the rigorous theory of complex analysis and the deep structure of analytic continuation, he could not pretend to understand.

[Sufficient for Physics calculations, insufficient in mathematical depth.]

[Numerical Methods and Computational Physics]

In the low-spec environment of the Wasteland, he had already formed practical abilities.

Finite differences, simple iteration, numerical stability, step-size selection, and error accumulation—he had suffered setbacks from all of them.

He knew that the calculation result was not the answer.

A program spitting out numbers did not mean the world had explained itself clearly.

The most important thing about numerical methods was not calculating the result, but knowing when it would diverge and when it was simply writing errors down with greater precision.

However, he lacked the conditions for systematic training in truly modern computational Physics, high-performance computing, and complex simulations.

He wrote down this item.

[Low-spec numerical modeling usable, modern computational systems require remedial lessons from reality.]

[Theoretical Mechanics]

The undergraduate core has been conquered.

Newton, Joseph-Louis Lagrange, and Hamilton were no longer three sets of formulas, but three heights from which to view the world.

Newton looked at local forces.

Joseph-Louis Lagrange took a step back to look at the degrees of freedom and energy structure of the entire system, not chasing constraint forces, but only pursuing directions that could truly move.

Hamilton took another step back to look at the overall evolution of states and paths, placing position and momentum on equal footing, turning mechanics into geometry in phase space.

Through the wind-powered water-lifting device, he understood for the first time what it meant not to chase the tension of every single rope.

He knew that generalized coordinates were not just changing names, but grasping true degrees of freedom.

He knew that the action taking a stationary value was not some crude chicken soup saying nature always chose the most economical path, but a variational condition concerning paths with rigorous mathematics behind it.

He could derive the Lagrange equations from the variational principle; they were not memorized, but truly understood in their variational logic.

He understood Hamiltonian mechanics; canonical equations, Poisson brackets, and canonical transformations—he had done problems on all of these and could use them.

There was a door he had always seen, but had yet to push open.

From Hamiltonian mechanics to the Hamilton-Jacobi equation, he had gone through that transition several times and could follow the steps.

But as for the connection between it and Quantum Mechanics—how the Schrödinger equation degenerated into the Hamilton-Jacobi equation under a certain limit—he only knew the conclusion and did not truly understand why.

Classical mechanics and Quantum Mechanics shared a secret passage here; he could touch the crack of the door, but had never gone inside.

[Theoretical Mechanics is no longer outside the door for him. But for deeper contents like canonical transformations, Poisson brackets, and symplectic geometry, he can only write: Know where the door is. Cannot pretend to understand.]

[Electrodynamics]

The undergraduate entrance had been passed.

Maxwells Equations were no longer unfamiliar.

Electric fields, magnetic fields, displacement current, electromagnetic waves, boundary conditions, and energy flux already formed a stable physical image.

Most importantly, fields were no longer treated as auxiliary tools.

Fields truly existed.

Sunlight fell onto the panel surface, energy propagated in the electromagnetic field, charges separated within the semiconductor, and current entered the storage battery along the lines.

He was connected to the entire universe by countless invisible electromagnetic lines, except that not a single line connected him to another person.

But the electromagnetic intuition given to him by the Wasteland was all low-frequency and quasi-static.

For true radiation, waves, and relativistic effects, he had no intuition, only derivation.

The derivations could keep up, but the intuition was empty.

He had studied relativistic formalism once.

He knew the gist of covariant writing, but tensor operations were never smooth.

Every time he handled a specific problem, he had to stop to confirm indices, symbols, and transformation relations; it was not smooth to the point of being automatic.

He had a framework for the radiation part, but he had never been forced by any practical problems in the Wasteland; this part probably remained at the level of having read and understood it without truly internalizing it.

This was the place among his four courses most likely to harbor large-scale pseudo-understanding.

Admitting this was slightly easier than pretending it didn't exist.

[Undergraduate electromagnetic field image formed; complex boundary values and higher-order theories need retaking.]

[Thermodynamics and Statistical Physics]

The core concepts had entered his life.

This course wasn't just something he had studied; it changed the way he understood himself.

Entropy increase, free energy, open systems, irreversible processes, equilibrium states, and fluctuations—these words were already bound together with his Stone House, farmland, body, memories, and notes.

He knew he wasn't conquering entropy increase, but merely maintaining himself and the camp as a local low-entropy structure by continuously inputting free energy.

Stopping maintenance would mean returning to the Wasteland.

At this level, it was no longer about knowing how to do problems, but an understanding ground into his very bones.

But he also remembered that farmland weed diagram—from neat farmland to overgrown weeds, with an arrow in the middle reading "the direction of entropy increase".

He drew it when he first studied statistical Physics in his thirty-fourth year, and even after finishing it, he felt he understood entropy.

That was just a diagram, not understanding.

It was the familiarity given by Wasteland experience that disguised itself as understanding.

The richer the Wasteland experience in a place, the easier it was to produce this kind of pseudo-understanding, because it felt right, so he didn't pursue it further.

The rigorous derivation of statistical Physics, partition functions, ensemble theory, and the complete system of quantum statistics could still not be said to have been completely conquered.

[Extremely deep thermodynamic concepts, statistical Physics computational systems need retaking.]

[Quantum Mechanics]

The first round of truly passing through the door.

Not mastery, but the door had been passed.

He no longer forced the world to answer using only classical causality.

He no longer understood all probabilities as "I don't know", nor did he treat measurement as passive reading.

Blackbody radiation, photoelectric effect, de Broglie waves, Schrödinger equation, one-dimensional potential well, harmonic oscillator, tunneling, and uncertainty—he could rebuild basic images, do problems, derive things, and explain clearly what every step was doing.

This wasn't that feeling of understanding from his twenty-second year, but a solidity ground out by hundreds and thousands of wrong problems, a door he took twelve years to walk into.

More importantly, Quantum Mechanics had pried open his underlying worldview, and he hadn't run away.

It loosened his entire set of worldviews relied upon for over twenty years—that there were always classical answers hidden behind the scenes—and he stubbornly stayed there without bypassing it or glossing it over with a pretty analogy.

This was very important.

The advanced parts were uneven; angular momentum and spin he understood, having done systematic problems.

Identical particles and quantum statistics he understood; when studying statistical Physics, he came back to walk through this area again, which was deeper than the first time.

Scattering theory he only had a framework for, the Born approximation was usable, but the S-matrix part he had only read without doing enough problems, so he was unsure of the depth of his understanding.

Quantum field theory—he wouldn't touch it; in this lifetime, he lacked the ability to walk into it alone.

The most important thing Quantum Mechanics taught him was not a specific concept, but something about learning itself.

Thinking one understood did not equal actually understanding.

After stumbling many times over this matter in Quantum Mechanics, he truly began using problem-solving to test understanding rather than relying on feelings.

This change was more important than any specific piece of knowledge.

[Worldview has passed through the door, basic models can be rebuilt, higher-order quantum systems require systematic remedial lessons.]

Jiang Lin wrote down item by item.

The more he wrote, the clearer-headed he became.

This table didn't look like a list of victories.

It looked more like a ledger of debts.

Where he owed debts, where he had merely passed through, where truly belonged to him, and where he had just been dragged hard across by time—he marked them out one by one.

If it were the twenty-year-old Jiang Lin, he probably wouldn't have been willing to write like this.

He would have felt embarrassed.

After decades of Wasteland career, searching high and low, how could there still be so many unfinished things, items needing retaking, and things he couldn't pretend to understand?

But the fifty-something Jiang Lin wouldn't think this way.

Forty years was very long.

Long enough to grind and flatten a person round and flat.

But forty years was also very short.

So short that when facing the entire human knowledge system, it was still like a tiny stub of a candle shining into a huge cave.

He was no longer that youth who thought that as long as there was enough time, he could crush everything.

Time was not omnipotent.

Time only gave one the qualification to make mistakes, correct them, make mistakes again, and correct them again.

Whether it could turn into something real also depended on aptitude, methods, tools, the body, the environment, and whether one was honest enough.

In the winter of his thirty-ninth year, Jiang Lin did a second thing.

Delete.

This matter was even harder than organizing.

He re-checked all the files on the computer that could still be read.

Many old files no longer had meaning.

Screenshots of wrong problems from his early years—delete.

Repeatedly downloaded textbook versions—delete.

Low-credibility experimental data—keep the index, but move it out of the main database.

Erroneous derivations—keep a few typical ones, delete the rest.

Emotional reviews—delete the majority.

Models that he had spent a lot of time building but later proved useless—keep the conclusions, delete the intermediate redundant files.

Every time he deleted a file, he had to restrain his impulse to keep it.

These were all things he spent time making.

Behind some files was a cold night.

Behind some was a failed experiment.

Behind some was sitting at the stone table all day doing proofs until his wrists ached.

Deleting them felt like denying his past self.

But Jiang Lin knew he had to delete them.

Hard drive space was not the only issue.

More importantly, a person's memory space was also limited.

If everything was treated as a treasure, truly useful things would be buried.

There was a sentence in his concept notebook that he had repeatedly written over the years.

[Complexity is not the goal; effectiveness is.]

Now it was his turn to execute this sentence on his own past.

Then, he began copying out the final index.

Writing only the index.

Under each core concept, a maximum of three lines.

Line one: What problem it solved.

Line two: The easiest mistake to make.

Line three: Where to enter when relearning in the future.

For example, [Entropy]

Solved: Irreversibility, maintenance of order, open systems.

Mistake: Calling life a violation of entropy increase; treating "chaos" as the sole explanation.

Entry point: Clogged filter tanks, equipment wear, farmland degradation, forgetting of the concept notebook.

For example, [Action]

Solved: Looking at motion from the entire path rather than just instantaneous forces.

Mistake: Calling stationary values always minimum; replacing mathematics with poetry.

Entry point: Lagrange equations, generalized coordinates, wind-powered water-lifting device.

For example, [Quantum State]

Solved: State description and measurement probabilities of microscopic systems.

Mistake: Forcibly searching for classical orbits; treating probabilities as mere ignorance.

Entry point: Double slits, photoelectric effect, one-dimensional potential well, uncertainty.

Such indices he wrote page by page.

Because every sentence had to be compressed, he wrote very slowly.

Compression was much harder than expansion.

Expansion could be wordy, could go around in circles, could pave the way with metaphors.

Compression required him to truly know which things were bones and which were merely scaffolds he had built back then to help himself climb across.

Sometimes, he would stop for half a day over a single word.

Were the two words "Solved" correct?

Was the entry point too confident?

Had the mistake clearly stated the problem?

If his eighteen-year-old body returned to reality in the future and relearned using these memories, would these three lines be enough to bring him back?

Not enough.

Of course not enough.

But at least they were signposts.

In the spring of his fortieth year, the Stone House experienced a moderate storm.

That made the roof rattle all night long.

Jiang Lin did not sleep well.

Waking up in the morning, he first checked the remaining brackets of the solar panels, then looked at the roof, then the water storage pit, and then the farmland.

There wasn't much problem.

A section of the roof drainage ditch was blocked by sand; clearing it would suffice.

An old crack on the outer wall of the storage room had expanded and needed repairing.

Two places of the farmland's covering layer had been blown off.

These checks he performed very skillfully.

As skillfully as breathing.

But after finishing, he stood at the doorway of the Stone House, looking at the Wasteland swept by the wind, and suddenly realized something.

He was soon going to leave this place.

This Wasteland, together with the previous times combined, was where he had lived for nearly fifty years.

It used to be fear, then an enemy.

Later still a test paper, and later still a system.

By now, it even felt somewhat like a silent old acquaintance.

Unfriendly, ungentle, and had never truly accepted him.

Yet it accompanied him through the longest stretch of his entire life.

Here were the farmland where he had failed, the Stone House where he had been frozen awake countless times, the ink pens he had written to ruin, and the pit where he had buried retired batteries.

There were also all his footprints from walking from eighteen to over fifty years old.

It was also in this year that he had to reduce the area of his farmland.

It wasn't because there was too much food, but because his body's accounts no longer balanced.

Turning a ridge of land used to just mean tiredness when he was young; now it meant a heavy lower back, sore knees, and his calves would cramp up in waves after lying down at night.

Waking up the next day, recovery was also slow.

The physical strength that used to recover overnight now took two or three days to slowly make up for.

He finally admitted that high-intensity cultivation of two hundred square meters no longer belonged to this body in its fifties.

The objective function changed from long-term maximization of output to stability, low maintenance, and low risk within the remaining years.

When this sentence was written into the document, Jiang Lin smiled wryly with helplessness.

As people lived later into life, even their objective functions would change.

When young, one pursued maximum values; in middle age, stability; in old age, maintainability.

In the late spring of his fortieth year, Jiang Lin put all the final indexes into a moisture-proof bag.

In the midsummer of his fortieth year, his body began to noticeably deteriorate.

Sometimes waking up in the middle of the night with a dry throat and a tight chest, even turning over required gathering a bit of strength first.

When getting up in the morning, as the soles of his feet touched the ground, his knees and waist would answer the weather for him first.

Sometimes walking to the edge of the field, he would forget which tool he had just wanted to grab.

In the winter of his fortieth year, he developed a low-grade fever.

At first, he thought it was just a common infection.

He handled it in the most conservative way—drinking water, staying warm, and reducing activity.

The next day, the fever subsided a little.

On the third day, it rose back up a bit.

Repeating like this, it was like a needle pricking into a leather pouch that had been leaking air for a long time.

Things that a young body could endure, at fifty-eight years old, only needed a gentle push to shove the entire system across the boundary.

He couldn't hold out anymore.

Jiang Lin opened the last page of his concept notebook.

Tremblingly, he wrote down—

[Fourth Wasteland, fortieth year.]

[I haven't finished learning.]

[But I have learned how to continue learning.]

[Going back.]

Finishing the last word, he put down his pen.

The fire in the stove was very small, and outside the window was the grey-white sky of late winter.

Jiang Lin leaned against the stone chair, hearing his breathing become slower and slower with each sound.

He lowered his head and glanced at his hands.

Old and rough, with deformed finger joints.

After going back, all of these would disappear.

The next second, darkness fell before his eyes.