From the book, the two learned that soul evolution was a qualitative transformation of the soul and soul power.
Once a soul evolved, the spiritual sense and soul power would extend further and become more penetrative; able to perceive things that ordinary spiritual sense and soul power couldn't.
Additionally, soul attacks and defense would become more than a thousand times more powerful and most importantly, ordinary cultivators wouldn't be able to sense spiritual sense probes by evolved souls.
However, Shen Xu and Zhen Yuan were more interested in another matter.
"What does it mean that only evolved souls can perceive the true nature of the universe?" Shen Xu wondered and Zhen Yuan shook his head.
"I don't know but it sounds more profound than simply comprehending the dao." He said and quickly continued.
"Soul evolution has twelve levels and the first is the green soul stage." He said and Shen Xu nodded.
