Su Yu didn't know Bai Qian's thoughts, and even if he did, he wouldn't care. So what if the Cultivation Technique was incomplete? He possessed peerless talent; he had nothing to worry about.
Besides, if this Saber Technique was truly unlearnable, the interface naturally wouldn't show any changes. As long as he reached the entry-level, he could just grind it out. After reaching Perfection, it would be even easier. By constantly Breaking Limits, he might end up even stronger than that mighty Cultivator!
