不得自專 The Son May Not Act Alone
行者道：「兄弟莫題。古書云：『父在，子不得自專。』師父又在此，誰敢先去？」行者道：「兄弟莫题。古书云：『父在，子不得自专。』师父又在此，谁敢先去？」
逐句解 Line by line
Pilgrim said, "Brother, do not raise it.
He refuses, which is the last thing needed to make his brother determined. 莫題, do not bring it up, is the same verb the narrator uses when dropping a thread of the story.
The old books say: while the father lives, the son may not act on his own authority.
子 is zǐ here, the son — a full noun in a classical quotation, not the weak zi that ends a colloquial word like blockhead. In 不得 the 得 is dé, the modal may. The shape is 不得 in front of a verb, not after one, and the two read differently. The line paraphrases a rule from the Book of Rites about a son's independence while his father lives.
And the master is right here. Who would dare go on ahead?"
He applies the classical rule to the pilgrimage, casting Tripitaka as the father. It is a real argument, correctly used, deployed entirely in bad faith.
Pilgrim quoting the classics to stop his brother doing the thing he wants his brother to do is the best-built move in the chapter. The refusal is what makes it irresistible.
It also puts the fault somewhere useful. If Bajie goes now, he goes in defiance of a quoted rule, which means that when he comes back beaten there is a ready answer for why he has no case.