念念那話 Sha Monk Proposes The Test
沙僧道：「施主休講。師父可坐在這裡，等我和二哥去，一家扯一個來到你面前，你就念念那話兒，看那個害疼的就是真的，不疼的就是假的。」三藏道：「言之極當。」沙僧道：「施主休讲。师父可坐在这里，等我和二哥去，一家扯一个来到你面前，你就念念那话儿，看那个害疼的就是真的，不疼的就是假的。」三藏道：「言之极当。」
逐句解 Line by line
Sha Monk said: "Benefactor, say no more. Master, you may sit here. Let me and second brother go, and each of us drag one of them before you; then you recite that spell of yours, and see which one suffers pain — that is the true one — and the one who does not is the false."
Sha Monk proposes the fillet test, having watched it fail. 那話兒 is a coy way of pointing at the spell without naming it, so nà plus the light er ending — but 那個害疼的 four characters later asks which one, and takes nǎ. Both readings of 那 stand inside the one sentence.
Tripitaka said: "What you say is exceedingly apt."
極當 takes dàng, fitting, apt — where the automatic reading gives dāng, the positional sense used at 當面 one passage earlier. The two sit here as a matched pair, one character doing two jobs a few lines apart.
Sha Monk's proposal is the Bodhisattva's fillet test exactly, down to the wording — 看那個害疼的就是真的，不疼的就是假的 reproduces her 看那個害疼的便是真，不疼的便是假 almost character for character. He was standing beside her when she tried it, and he watched it fail.
That he proposes it anyway is not stupidity. It is the only test anyone has, and the reasoning behind it is sound: the fillet was fitted to one skull. What Sha Monk has not worked out — what nobody in the story works out — is that a test which failed in the Bodhisattva's hands will not succeed in Tripitaka's merely because the master is the one who put the fillet on.
Tripitaka's 言之極當 is a four-character endorsement of a plan the reader already knows will fail. The chapter keeps arranging for its characters to agree enthusiastically about the wrong thing.