It comes from a common proverb.
Book III of the Common Proverbs: Popular Compilation: Zhu and Shu: The sage opened the eight characters, but people themselves don't understand. "According to today's statement, there is a proverb called' eight characters don't see two', which seems to be because of this. People who have no end to everything are said to have never seen both sides of the eight characters. "
The meaning of this passage is that the door of saints has long been opened, but people have ignored it. They didn't go in, but went out. According to the existing proverb, eight words are inseparable, which seems to be the reason. People who have no clue about everything have never seen the so-called eight characters.