相剋之計 The Principle of Mutual Conquest
行者道：「我怎麼著忙？」沙僧道：「那妖精手段不如你，槍法不如你，只是多了些火勢，故不能取勝。若依小弟說，以相生相剋拿他，有甚難處？」行者聞言，呵呵笑道：「兄弟說得有理，果然我們著忙了，忘了這事。若以相生相剋之理論之，須是以水剋火。卻往那裡尋些水來，潑滅這妖火，可不救了師父？」沙僧道：「正是這般，不必遲疑。」行者道：「你兩個只在此間，莫與他索戰，待老孫去東洋大海求借龍兵，將些水來，潑息妖火，捉這潑怪。」八戒道：「哥哥放心前去，我等理會得。」行者道：「我怎么着忙？」沙僧道：「那妖精手段不如你，枪法不如你，只是多了些火势，故不能取胜。若依小弟说，以相生相克拿他，有甚难处？」行者闻言，呵呵笑道：「兄弟说得有理，果然我们着忙了，忘了这事。若以相生相克之理论之，须是以水克火。却往那里寻些水来，泼灭这妖火，可不救了师父？」沙僧道：「正是这般，不必迟疑。」行者道：「你两个只在此间，莫与他索战，待老孙去东洋大海求借龙兵，将些水来，泼息妖火，捉这泼怪。」八戒道：「哥哥放心前去，我等理会得。」
逐句解 Line by line
The Pilgrim said: ‘How am I in a fluster?’
Wukong bristles, which is exactly what Sha wanted -- it gets him to ask the question that lets the explanation happen.
Sha the Monk said: ‘That monster's fighting-skill is no match for yours, his spear-play no match for yours; it is only that he has the added force of the fire, and so you cannot prevail.
Sha lays it out in order. Not his skill, not his spear -- just the fire, added on top. He has isolated the single thing that changed the outcome, which is the sort of thinking neither brother has managed in three passages.
If you'll take your little brother's word, seize him by the principle of mutual generation and mutual conquest — where is the difficulty in that?’
And the answer: 相生相剋, mutual generation and mutual conquest. This is the other half of the Five Phases. The fire set-piece explained the generating half, where each phase produces the next in an endless ring. But the same system has a second ring running the other way, in which each phase defeats another: water puts out fire, fire melts metal, metal cuts wood, wood breaks earth, earth dams water. If you know the first ring you already know the second. 有甚難處 -- where is the difficulty? 甚 is shén again, the questioning sound.
The Pilgrim, hearing this, laughed aloud: ‘Brother, what you say has reason — indeed we got flustered and forgot this very thing.
Wukong concedes at once and without sulking, which is one of his better qualities. 著忙了, we got flustered, is him accepting Sha's diagnosis word for word, including the word he objected to a moment ago.
If we reason it by mutual generation and mutual conquest, then it must be water conquering fire.
以水剋火, water conquers fire. Four characters, and it is the answer to the entire elaborate rhapsody that exalted the fire as the engine of creation. The demon's own cosmology contains the instructions for beating him.
But where shall we find water to douse this demon-fire, and so rescue our master?’
Principle settled, Wukong moves straight to logistics, which is very much his mode. 那裡 is nǎ lǐ here, where -- a real question this time. Where do we get water?
Sha the Monk said: ‘Just so — no need to hesitate.’
Sha agrees and tells him not to dither. Again this is ordinary speech, not a set-piece cue.
The Pilgrim said: ‘You two stay here, and do not challenge him to battle; wait while old Sun goes to the Eastern Ocean to beg the loan of dragon-troops, to bring some water, douse the demon-fire, and take this insolent fiend.’
The plan, fully formed: stay here, do not pick a fight, and I will go and borrow the sea. 龍兵, dragon-troops -- he is not fetching a bucket, he is calling up an army from the Eastern Ocean. That is the scale Wukong operates at, and it is going to be nowhere near enough.
Eight Rules said: ‘Go with an easy mind, brother; we understand.’
Bajie agrees cheerfully. 理會得 means to understand or grasp, with the light 得. Whether he does is another matter.
Sha Monk supplies the missing half of a doctrine the chapter has already taught. The fire set-piece walked through mutual generation, the ring in which wood makes fire and fire makes earth and so on round. But the Five Phases have a second ring, mutual conquest, running in the other direction, and in that one water defeats fire. It is a matched pair, and anyone who knows the first knows the second.
The structural joke is neat: the demon's power was explained to the reader in a lavish poem about the Five Phases, and the Five Phases are exactly what undo him. His own cosmology holds the counter-argument. He simply never expected anyone to look it up.
The quiet disciple out-thinks the two fighters, and Wukong, to his credit, accepts it instantly and starts planning. What none of them has noticed is Sha's other point -- that the fire is not ordinary fire. Water conquers fire. Whether water conquers this fire is a question nobody asks.