When it came to seeing, and how to see, it was an extremely serious academic question.
Simply put, this world was filled with information, but every species received that information differently.
If eyes were compared to radios, then information was the frequency. For humans, perhaps the frequency their eyes received from birth was the 92.7 traffic broadcast channel. Naturally, what they saw was the information on that frequency. For ghosts, however, perhaps the frequency they received was that of the 105.2 music station, so they could not receive the traffic broadcast frequency. Thus, humans and ghosts could not see each other, much less communicate.
Of course, a very small number of people could switch the frequency their eyes received between 92.7 and 105.2. These were the so-called Yin-Yang Eyes. When tuned to the former, they received the traffic broadcast like everyone else. When tuned to the latter, they received the music broadcast.
As an Elder God, Duanmu Huai theoretically possessed the ability to cover every channel. However, just as adjusting a radio produced nothing but static except on a fixed channel, during the process of switching frequencies, Duanmu Huai also could not properly perceive the world. After all, his soul was human, not that of an Old God. For Duanmu Huai, viewing the world on the 92.7 frequency had become a habit. Unless the situation was special, he rarely chose to receive information from other channels.
That was also why Duanmu Huai could not lock onto that monster. Simply put, it was like tuning a radio, only no matter how you adjusted it, all you got was static. Without a way to accurately determine the frequency the other party was on, naturally, he could neither lock onto nor see it.
Or perhaps Duanmu Huai still had not figured out whether