The Magic Tower's 1st Floor Trial simply measured one's magic energy, or 'mana'.
Since casting magic required a significant amount of mana, only those who possessed this quality would be able to become mages, naturally limiting how many there were.
Higher-level spells took even more energy to cast, as well as a greater level of knowledge and comprehension, so those who could reach higher peaks were even less.
The 1st Floor Trial served as a way to quickly cull those who didn't meet the very first and most basic of many prerequisites.
Magic doesn't exist on Earth as it does in this Otherworld, but I'm not sure about the existence of magical energy. I would imagine that it doesn't exist on Earth, as otherwise it would have been discovered and researched by the scientific community, however.
So, ordinarily, I shouldn't have been able to pass the test, as I shouldn't have any magical energy in my body.
But, thanks to Reality, I was able to.
