Inscrutable

"Well, I can't help it, you know?" The man grumbled. "I'm no scientist. I just feel my way through these things."

"Master Reina had a very broad and thorough framework for her field that distilled her understanding of assassinations," Rui insisted. "Why don't you have one for mental techniques."

"Hah, assassinations are super simple and easy. Don't compare them to the most complex and inscrutable existence in this universe." The man snorted.

"Which is...?"

"The mind, young man, the mind." He got up, tapping the side of his head. "We know very little to nothing about how it works. Most mental masters rely on the fact that they too have minds to get past a lack of theoretical understanding and gain a more instinctual understanding or an understanding through lots of trial and error. But as for the core theory..."

He shrugged. "Perhaps only Martial Sages and Martial Transcendents understand more about it."