If you woke up one day and felt that something was wrong, what would you do? And when I say, something was wrong, I mean something was wrong with the world on a fundamental level.
Perhaps the sky was green but you remembered it was blue. Perhaps your father died before you were born but you saw him taking milk out of the refrigerator. Perhaps your friends in school thought you were cool when you remembered being bullied or perhaps you got picked first in kickball even though you were a scrawny little nerd. If something like that happened, and you began to question where and who you were, perhaps you would ask if you were still in the same universe.
But how does one confirm if the universe they woke up in today was the same universe that they went to sleep in the night before? Could one trust their own memory? Surely, not, but then what else could they rely on? Anything they could do to try to compare this universe with their previous universe would assume their memory of their old universe was accurate.
So, okay, we wouldn't get anywhere if we didn't at least assume that our memories were accurate. But if one wasn't sure what exactly they remembered, perhaps if their memories of the blue sky were as vague as their memories of a green one, then what could they do? The answer is to compare as many things as one can. If differences could not prove something, then perhaps similarities could.
One must measure the acceleration of gravity and compare it to what one remembered from the other world. And when one is convinced that it is the same, that is an important piece of data. One must measure and calculate the speed of light, elementary charge, electrical resistivity and heat capacity of various substances, and when they are the same, one can know that they have more evidence.
Outside of physics, one can study human bodies, and find the similarities in organs and structure. One can observe human social behavior, and conclude that it is similar. One can examine economics, history, art, and the form and structure of stories. One can zoom in on a piece of dust, taste a sip of water, smell the scent of leaves and grass, and one can find more evidence of the similarities between their memories and the world around them.
But is that enough?
No.
Even if all the physical constants were confirmed, even the one that was named after Planck in my previous world, even then, it would not be enough. Even if the grass, the water, the people, the scents, the tastes, the feelings of love and joy and happiness in your heart, even if they were all the same it would not be enough. Because all that would prove is that this universe follows the same laws and principles as your previous universe. All this would prove is that this second universe is similar to the one you remember, but that does not mean that it is the same.
And how could it be? After all, you can see your face in the pond, pointy ears and all. You can summon fire on your fingertips. Call forth the rain with a deep breath. Ride on a monster. Make the ground cave in. There are spirits that hover over the ground and fairies that walk around decked out in armor. There are demons and beastmen and even prehistoric humans. And of course, there are Immortals. Immortals taken straight out of myths and legends and poems that you only read once before.
An Immortal in a red star, named after a Mesopotamian superstition. An Immortal made of thirty birds, each one you know you should remember, but all you can remember is that 'thirty birds' is a terrible pun and the Simurgh sounded more mystical in Ancient Persia.
And what of Madness? Explored by Sophocles and Shakespeare and Gilman and many more, madness, with a small m, is an ancient theme. In my day, it had become more scientific with the advent of neuroscience, psychology, and many other scientific disciplines pertaining to the mind and the brain. But once upon a time, madness was a thing all on its own. Caused by demons or avarice, an infliction sometimes tangible sometimes ethereal. Madness was powerful, and in art especially so. Or so said Oedipus and Lear and the woman stuck in a room with the yellow wallpaper.
While developing my new magic system, I was thankful that I did not need to verify if this universe was the same as my own. Instead, all I needed to do was to confirm to myself, as much as possible, that this world followed, by and large, the same fundamental principles as my old Earth.
I had to confirm the physical constants as best I could, and I did that.
I had to confirm the basic principles of biology, and I did that.
I had to confirm the chemical properties of various substances, and I did that too.
And I had to confirm the way war and trade and love and compassion worked between sentient beings. The way any feeling being, sentient or not, reacted to stimuli, both positive and negative. I had to confirm if water was wet and the scent of cut grass was pleasant and the feel of sand was coarse between my fingertips as I remembered.
I had to remember what it felt to spend time with my friends, to feel the warmth of companionship, the love that fills the heart and empties the mind of idle worries and pointless thoughts. I had to confirm many things, some fundamental, some a little more complex, but after all this time in this world, and after meeting so many of the people of this world, I was fairly certain I had done my due diligence. I had confirmed, to a reasonable degree, that this world could be understood through the lenses I had acquired in my previous world. That it was reasonable to assume that something would work the way it did in my previous world, unless there was evidence to the contrary.
Where my old magic system required evidence to justify my beliefs, my new magic system already presupposed the evidence based on how similar this world was to my old one. My new magic system unshackled me from the burden of having to do a new experiment for every spell, to make an argument for why something that I had learned in school must be reinvented in this new world before I could make use of it.
After all, I had remembered that indeed, there had been other people who had come up with magic in this world. The elders of the elfin Jora tribe had used magic. Weak magic that they had barely conjured up after centuries of lived experience, but magic nonetheless. Centuries of knowledge and wisdom had come together to let the elders tell stories through fire and to keep fires going when they should have died out.
And yet there were beings in this world who used written words. The demons, for example, wrote things down on scrolls that they passed down to their future generations. Was that not an accumulation of knowledge like that of the elfin Jora tribe's elders? It was, indeed. But it was not an accumulation of wisdom. That, for most people, came from time.
All of these thoughts and considerations brought me back to myself. Me—the human from another world. In this world, I could invent spells that nobody else could come up with. At first, one might assume this meant there was something special about me and that this 'something' was related to my status in this world.
I had been summoned by the Immortal of Evil, the Evil Eye. I had been blessed and supported by the Immortal of Desire, the Simurgh. And I had been manipulated and visited by the Immortal of Madness. But I was sure it was not the Immortals that had made me special, because then they would have tried to control me. I also knew that although the struggle between these three powerful beings was probably what was keeping me free from their domination, their struggle could not give me the power to come up with spells.
What was special about me was the stuff I had learned on my Earth. A twenty-first century liberal arts education. From grade school to college, I had learned things in institutions, through books, and through the internet and my peers. And although I didn't have the 'wisdom' of years, the way the old elfin Elders did, I had the 'wisdom' of having done experiments in science labs, conducted research of my own, or seen results replicated in papers, movies, and real life. The complexities of modern life, with its ups and downs, the constant bombardment of information and stimulation. Of learning things, unlearning others, and having access to the collective knowledge of generations of humans at my fingertips. Everything had given me the 'knowledge' and the 'wisdom' that was necessary to create magic in this world. All I'd needed to do was to convince myself that this world and the world I had been born in, were not that different.
That here too, I could be guided by the motto of my alma mater: Truth.