一句話：「自中丁以來，廢適而更立諸弟子，弟子或爭相代立，比九世亂，於是諸侯莫朝。」九位商王的政治變動，被壓成這一句因果：嫡子被擱下，改立弟與子；弟與子又爭著上位；亂了九世；諸侯不來了。麻煩在於，同一篇的世系表裡，這句話之後還接著記「弟及」。
一篇世系表的寫法
《史記·殷本紀》一王一句往下排：「湯崩，太子太丁未立而卒，於是乃立太丁之弟外丙」；「帝外丙即位三年，崩，立外丙之弟中壬」；「帝中壬即位四年，崩，伊尹乃立太丁之子太甲」——太甲是「成湯適長孫」。再往下：太甲崩，子沃丁；沃丁崩，弟太庚；太庚崩，子小甲；小甲崩，弟雍己；雍己崩，弟太戊；太戊崩，子中丁。弟及與子繼從一開始就交替出現，不是某一天才改的。
中丁到陽甲這一段，是「九世」的所在。逐句看：中丁崩，弟外壬立；外壬崩，弟河亶甲立；河亶甲崩，子祖乙立；祖乙崩，子祖辛立；祖辛崩，弟沃甲立；沃甲崩，立沃甲兄祖辛之子祖丁；祖丁崩，立弟沃甲之子南庚；南庚崩，立祖丁之子陽甲。中間還夾著評語：「河亶甲時，殷復衰」「帝陽甲之時，殷衰」。然後才是那句總評。
「九世」與「亂」
「九世」是哪九世？原文沒有點名。按世系數，從中丁到陽甲正好九王；這個數目是讀者排出來的，不是原文給的名單——所以本文只說「自中丁至陽甲，凡九王」，不把它當成《史記》的正式說法。
「廢適而更立諸弟子」是解釋句：把嫡子（適）擱下，改立弟、子；下一句「弟子或爭相代立」，說的是爭位。要注意這是《史記》作者的評語，不是商代的法令、盟誓或即位記錄；這一句能證明的是「史書這樣解釋」，不是「商人這樣規定」。
更要緊的是下半段：這句話之後，陽甲崩，弟盤庚立；盤庚遷殷、行湯之政，「殷道復興」；盤庚崩，弟小辛立；小辛崩，弟小乙立；小乙崩，子武丁立。再往後，祖庚崩，弟祖甲立；廩辛崩，弟庚丁立。也就是說，「九世之亂」之後，弟及照舊出現。所謂「亂後改為父死子繼」，這篇世系表並不支持；「亂」的界限只是那句評語。
周人另外一套說法
另一部書給出的是另一種數。《尚書·無逸》裡，周公對成王說：「昔在殷王中宗，嚴恭寅畏，天命自度，治民祗懼，不敢荒寧。肆中宗之享國七十有五年。其在高宗……肆高宗之享國五十有九年。其在祖甲……肆祖甲之享國三十有三年。自時厥後，立王生則逸……亦罔或克壽，或十年，或七八年，或五六年，或四三年。」——三位好王享國長，之後諸王享國短，短到三四年。
這段話的說話人是周公，對象是成王，殷的往事是他勸誡的素材：他在講「無逸」，不是在講繼統法。把它當商代制度依據，會讀錯方向。而且兩頁在「祖甲」上直接衝突：《無逸》把祖甲列為四位「迪哲」（由哲）之一，享國三十三年；《史記》卻說「帝甲淫亂，殷復衰」。同一個王，一處是明君，一處是亂主。本文照錄兩邊，不裁決。
能寫與不能寫
所以有三條不能寫：不能寫「商朝實行兄終弟及制」——這裡只有一篇世系敘述；不能寫「九世之亂後確立父死子繼」——同篇亂後仍記弟及；不能把《無逸》當商代檔案——它是周公勸誡。能寫的是：這本書這樣記，並給出「廢適更立諸弟子」這個解釋；周人這樣數享國長短。
還有一層：「九世之亂」是高度概括的說法。它把九王的政治變動壓成一條因果鏈（廢適→爭立→亂→諸侯莫朝），而每一次具體怎麼回事——誰爭、誰立、有沒有外服插手——全都沒有。它像一句史論，不像一份記錄。
材料沒有的
沒有商代自己的繼統法文本；沒有「九世」的正式名單；沒有每次繼位的原因；沒有諸侯「莫朝」的實證；甲骨周祭譜系可以核對先王次序，但不在本文所據兩頁之內，這篇不引。所以這篇文章寫的是「一本書怎麼講商代繼承」，不是一部商代繼承制度史。
延伸閱讀：五十名博士弟子：漢太學的初置 · 「卜其晝，未卜其夜」：一場酒、一個孩子與兩套問法 · 書同文字：一句詔令，和三百多年後的解釋
參考來源與延伸閱讀
- 司马迁《史记》卷三·殷本纪（西汉；维基文库本，未与点校本复核） — The only one of these two pages that narrates Shang royal succession. It runs king by king in a fixed formula — 'X died; his son Y was set up', 'X died; his younger brother Y was set up' — and the alternation begins at the start: Tang's heir Tai Ding died before reigning, so Tai Ding's younger brother Wai Bing was set up; Wai Bing died and his younger brother Zhong Ren followed; Zhong Ren died and Yi Yin set up Tai Ding's son Tai Jia, the eldest grandson. From Zhong Ding to Yang Jia the entries give a run of younger-brother and nephew successions, after which the chapter says: 'From Zhong Ding onward the legitimate heir was set aside and younger brothers and sons were set up instead; the younger brothers and sons sometimes fought to succeed one another, and for nine generations there was disorder, so the lords no longer came to court' (自中丁以來，廢適而更立諸弟子，弟子或爭相代立，比九世亂，於是諸侯莫朝). The same chapter then continues to record succession by younger brother after this passage — Yang Jia to Pan Geng, Pan Geng to Xiao Xin, Xiao Xin to Xiao Yi — and again later (Zu Geng to Zu Jia, Lin Xin to Geng Ding). It gives no law of succession, no reason for any individual accession, and no list of the 'nine generations'.
- 《尚书·无逸》（周初；维基文库本，未与点校本复核） — A speech by the Duke of Zhou to King Cheng of Zhou, not a Shang document. He recalls the Yin kings: 'Formerly, in the time of Zhong Zong of Yin, he was grave and reverent, revering Heaven's mandate, governing the people in awe and daring not to indulge in idleness; thus Zhong Zong held the state seventy-five years. As for Gao Zong, he had long laboured outside and come to know the lesser people; when he took the throne he kept silent for three years… thus Gao Zong held the state fifty-nine years. As for Zu Jia, he did not think himself fit to be king and had been a lesser man; when he took the throne he understood the people's reliance and could protect and benefit the common folk, not daring to insult widowers and widows; thus Zu Jia held the state thirty-three years. After that time, the kings who were set up were idle from birth… and after that time none could reach old age: some ten years, some seven or eight, some five or six, some three or four' (周公曰…肆中宗之享國七十有五年…五十有九年…三十有三年…或十年，或七八年，或五六年，或四三年). The speech gives reign lengths, not rules of succession, and it names three good kings and 'our King Wen' as the four who followed wisdom.
