肉眼凡胎 Mortal Eyes Cannot Tell
長老立定腳，問他一句道：「女菩薩，你有甚事，綁在此間？」咦！分明這廝是個妖怪，長老肉眼凡胎，卻不能認得。长老立定脚，问他一句道：「女菩萨，你有甚事，绑在此间？」咦！分明这厮是个妖怪，长老肉眼凡胎，却不能认得。
逐句解 Line by line
The elder planted his feet and put a question to her: "Lady Bodhisattva, what has befallen you, that you are bound here?"
Another bare question-word 甚, read shén. Calling a stranger 女菩薩, Lady Bodhisattva, is simply courteous monastic address for a laywoman — but the book is going to keep using it for a demon for the rest of the chapter, which after a while stops sounding polite and starts sounding like a joke at his expense.
Well now!
One character, and the narrator uses it to turn to the reader before delivering the bad news.
Plainly this creature was a demon, and the elder, with his eyes of flesh and his common womb, could not tell.
肉眼凡胎 — eyes of meat, an ordinary womb — is the standard phrase for merely mortal perception, and it is not really an insult. It is a statement of equipment. Tripitaka is a man; men cannot see through this.
The narrator breaks in and tells us outright. There is no suspense to be had here and the book does not want any, because the interest of the next several pages is not in what she is but in whether anybody will be believed about it.
The phrase about eyes of flesh is doing careful work. It excuses Tripitaka completely, and in doing so it sets up the real charge against him. He cannot be blamed for failing to see through her. He can be blamed for what he does when somebody who can see through her tells him so.