Runes were unlike normal letters that everyone was accustomed to. They possessed power, and to untrained eyes, they would all look the same to some degree.
One needed to know them to spot the differences and even use them. Just like now, Eugene only knew the Rune of Summon because that knowledge had been inserted into his mind completely.
Wherever he saw it, he would recognize it. But any other runes were just the same to him. He had to look and feel them.
You don't read runes—you feel them, then read them, then comprehend them. If that happens, then even with your eyes closed, a rune close to you will appear like a spark.
However, learning wasn't easy. Even with the Mystery Door that gave Eugene access to many runes to explore, it would take him decades to understand them at a basic level, let alone use them.
