Edition note: An independent English rewriting of the Chinese argument, shaped for English-language readers.
Cheng Lingsu is the most intelligent person in the novel. The Poison Hand Medicine King leaves his medical and toxicological knowledge to this plain, slight, yellow-faced youngest disciple rather than to the seniors who later return to seize it. Alone in the herb cottage, she defeats them with judgment and pharmacology rather than force.
Hu Fei seeks her help for Miao Renfeng's poisoned eyes, and she joins his journey. She loves him; he loves Yuan Ziyi. Cheng understands the arrangement earlier and more completely than he does. Instead of competing, she proposes that they become sworn siblings. The name creates a place from which she can remain beside him, but it also closes the other possibility by her own hand.
She accompanies his pursuit, prepares remedies, detects attacks he never notices, and listens when he speaks of Yuan. She makes no claim in return and converts none of her care into a debt. That restraint prevents her devotion from becoming another structure surrounding his choices.
At the end, Hu is poisoned by three substances that cannot be neutralized together. Cheng knows one method: draw the poison out through the wounds, transferring it to a body that cannot survive. She renders him unconscious and acts. He does not know what she is doing and cannot say no.
The scene hurts because her gift is entirely her own and still decides another person's life without his consent. She has never tried to govern him; in her final act she chooses for him once, deciding that he must live after she is gone. A voluntary sacrifice can be free on the giver's side while closing freedom on the receiver's.
The novel surrounds her with people taken and used by power, rank, kinship, and desire. Cheng is not taken. She gives herself for a reason that belongs to her. That distinction matters, but it does not make the act simple. Hu wakes only after every decision has been completed. He buries her and continues alone, carrying a life he did not ask her to purchase.
Based on the Sanlian Bookstore edition of Jin Yong's The Young Flying Fox; later revisions differ in some details.
版本说明：中文原作；简体与繁体同页切换。
程灵素这个名字是她师父取的，出处是两部古医书：《灵枢》和《素问》。
她是毒手药王无嗔大师收的最后一个弟子，也是最小的一个。师父把一身的医术和毒术都留给了她，没有留给前面那几个。
那几个人后来都回来找她。师叔石万嗔，师兄慕容景岳，师姐薛鹊——他们要的是师父留下的东西。她一个人在药庐里，把这几个人一个一个打发掉了，靠的不是武功，是脑子和药。
这本书里她是最聪明的一个。
她相貌平常。书里写得很直接：她不好看，个子小，脸黄。她自己知道，也从不在这件事上花心思。
胡斐是为了苗人凤的伤找到她的。苗人凤被田归农的一封毒信瞎了眼睛，那个毒只有毒手药王一门能解。
她跟着胡斐上了路。
后来的事情，书里写得不动声色：她喜欢他，他喜欢的是袁紫衣。
她看出来了。她没有争，也没有说破，她做的是另一件事——她跟胡斐结为义兄妹。
她给这段关系找了一个能一直待下去的名分，代价是把另一种可能亲手关掉。
结拜之后她还是跟着他走。他去追凤天南，她跟着；他要救苗人凤，她配药；他闯天下掌门人大会，她在旁边接招。他想袁紫衣的时候，她在旁边听着。
她没有一次跟他要过什么。
书里有几处写她一个人的时候。她替他配药，替他想办法，替他挡下一些他不知道的暗算。这些事她做完就过去了，也没有拿出来讲过。
最后一次是中毒。
胡斐中了三种剧毒，这三种毒配在一起无药可解。她懂这一门，她知道剩下的只有一个办法：把毒从伤口里吸出来。
吸的人自己会中毒，而且没得救。
她把他弄晕了，然后动手。
她做这件事的时候，那个人是昏着的，不知道，也没有机会说不要。
她吸完了，替他敷上药，交代了后事，然后死了。
胡斐醒过来的时候，一切都办完了。
这一段在书里读起来很难受，难受的地方不只是她死了。
她这一生做过的事情排一排：她替一个不喜欢她的人配药、挡毒、想办法、陪着走了很远的路，最后拿命把他换了回来。她从头到尾没有向他要过一句话，一个承诺，一个交代。
她也没有告诉他这件事有多重。她连让他有机会拒绝都没有。
她这一生唯一一次替别人做主，是替他决定了他要活着。
这件事跟这本书里的另外几件放在一起看，形状是清楚的。
凤天南对钟阿四一家用的是权势；福康安对天下掌门人用的是名分和赏赐；袁紫衣的师父给她的是一份连顺序都排好的差事；南兰和马春花被两个男人的话定住。
这些都是从外面加上去的。
程灵素这一件不是。没有人要求她去死，也没有人给她任何回报。她做那件事的理由只在她自己那里。
同样是把自己交出去，是别人拿去用的，还是自己拿去给的，是两件事。
书没有替她评一个字。它只写了她吸毒的时候胡斐是昏着的，写了她交代完后事就走了，写了胡斐醒来之后知道了这件事。
后来胡斐把她葬了。
再往后他还在江湖上走，追他要追的人，做他要做的事。这本书结束的时候，他一个人。
那一年他二十来岁。
注：本文所据为金庸《飞狐外传》三联书店版。人物、情节与引述均出自该版本；新修版在若干处另有改动，与本文所论不涉之处不另作说明。