The moment I opened my eyes, I thought something was wrong with my vision.
I couldn't see anything.
No.
That wasn't quite right.
I could see.
It was just that the entire world had become a single line.
Forward and backward.
That was all there was.
No right. No left.
No up. No down.
I closed my eyes, then opened them again.
Nothing changed.
A single line stretched endlessly before me.
I turned around.
The same line continued behind me, disappearing into infinity.
"...A dream?"
I spoke aloud.
I could hear my own voice, but I couldn't tell where the sound was spreading.
In fact, the very concept of something "spreading" didn't seem to exist in this world.
I tried to reach out my hand.
That was when a chill ran through me.
I had no hand.
No legs.
No torso.
And yet, somehow, I still existed.
It felt as though my entire body had been crushed down onto a single line.
I thought about moving forward.
And I moved.
There was no sensation of walking.
I simply slid forward along the line.
After traveling for a while, I noticed something ahead.
A black dot.
As I drew closer, it began to move.
"Is someone there?"
The black dot stopped.
Then a voice answered.
"Who are you?"
It was a human voice.
"I don't know either. I just found myself here."
"Here?"
The other person fell silent for a moment.
"Is there somewhere other than here?"
I didn't understand the question.
"The world I came from. The normal world."
"Normal?"
"You know... a world with up and down, right and left."
A long silence followed.
Then the voice said,
"What are you talking about?"
I was speechless.
"What is 'right'?"
It wasn't a joke.
I could tell from the voice that the question was completely serious.
"Then... what about up?"
"I don't know."
"Down?"
"I don't know."
That was when I finally understood.
To the inhabitants of this world, nothing existed except forward and backward.
It wasn't that they didn't know about other directions.
The concepts themselves didn't exist.
I tried to pass the black dot.
I couldn't.
It was directly in front of me.
Without a sideways direction, there was no way to go around it.
"Could you let me pass?"
"Where?"
"From the side."
"What's a side?"
Again.
The same problem.
With no other choice, I asked the other person to move backward, and we continued along the line together.
I had no idea how much time passed.
There was no sun.
No clock.
No changing scenery.
Only a single line stretching endlessly into eternity.
Then something happened.
Something vibrated inside my pocket.
Except I didn't have a pocket.
I barely even had a body.
And yet, somehow, I knew exactly what it was.
My smartphone.
The next instant, words appeared before my eyes.
DAY 1
DIMENSION: 1
I held my breath.
More words appeared.
[DIMENSIONAL TRANSFER PROGRAM INITIATED]
"...Dimensional transfer?"
I whispered.
The black dot beside me responded.
"What are you talking about?"
Apparently, I was the only one who could see the words.
The display changed.
[QUESTION]
[NAME ONE THING THAT DOES NOT EXIST IN THIS WORLD.]
Something that doesn't exist?
I thought about it.
Almost nothing existed here.
No sky.
No ground.
No buildings.
Even human bodies appeared as little more than points.
But somehow, none of those felt like the answer.
"Height."
[ERROR]
Wrong.
"Width."
[ERROR]
"Depth."
[ERROR]
What was it?
None of those things existed here.
Then I remembered my conversation with the inhabitant.
What is right?
What is up?
They weren't simply unaware of those directions.
They couldn't even imagine a direction that didn't exist.
What if I could somehow leave this single line?
What if I could look down on this world from above?
Then I could see the entire line at once.
But as long as I remained inside this world, I could never reach such a place.
I gave my answer.
"Outside."
For several seconds, nothing happened.
Then the words changed.
[CORRECT]
[ONE-DIMENSIONAL WORLD CLEARED]
The next instant, the world trembled.
The single line suddenly split into two.
No.
That wasn't it.
The line hadn't multiplied.
A new direction had been born into the world.
My body was pulled away from the line.
It was a sensation I had never experienced before.
Sideways.
I was moving sideways.
Into a direction that, only moments ago, I hadn't even been able to comprehend.
The world expanded rapidly.
The line became a plane.
The inhabitant who had appeared as a black dot began transforming into a flat shape.
Then new words appeared before me.
DAY 2
DIMENSION: 2
Below them, a warning appeared in red.
[WARNING]
[IN THE TWO-DIMENSIONAL WORLD, YOU CANNOT HIDE WHAT IS INSIDE YOU.]
"Inside...?"
Before I could understand what it meant, something appeared in front of me.
It had the shape of a human being.
But it was completely flat, like a figure drawn on a sheet of paper.
The moment the being saw me, it froze.
Then, in a trembling voice, it said,
"You..."
"What's wrong?"
The flat human stared directly at the center of my body.
"Who is the other person inside you?"