家庭背景

鄭覃的生年不詳，但他出生於一個顯赫的家庭，父親鄭珣瑜在唐德宗、唐順宗父子年間任宰相。

仕途生涯

在步入仕途前，一開始在弘文館擔任校書郎的職務，此後也當過像拾遺、補闕這樣的小官，擔任補闕時，與崔植同僚且同為賢相之子，每朝廷有得失，兩人都上疏論執，譽望蔚然。

之後又被借調去擔任考功員外郎與刑部郎中。

元和十四年（819年）二月，鄭覃被任命為諫議大夫。當唐憲宗想派五位宦官為京西北和糴使率軍運糧到西北的回鶻、吐蕃邊境時，鄭覃上疏表示反對，唐憲宗便取消了這項命令。

元和十五年（820年），唐憲宗駕崩，唐穆宗繼位。但唐穆宗喜好享樂。十月，在閣內召集群臣。鄭覃和崔郾、崔玄亮等四名同僚趁機進諫，指出穆宗把太多的時間花在了飲宴和遊樂上，在賞賜寵臣時耗費了國庫太多的金錢。穆宗很驚訝，起初感到不悅，問宰相蕭俛這是誰的上書。蕭俛說這是諫議大夫們的主意。穆宗好言撫慰了他們，但並沒有將進言的內容付諸實際，只是稍減畋獵。當時久無閣中奏事，鄭覃等有所抗論，人們都相慶賀。鄭覃也得以賜爵。

當月稍早，長期對抗朝廷的成德（軍部在今石家莊）節度使王承宗去世了，成德軍想擁立其弟王承元為新的節度使，但王承元想把成德軍的軍權交還給朝廷。成德軍于是不讓他就任義成（軍部在今安陽）節度使。王承元請求穆宗派一位重臣來勸士兵就範，于是十一月穆宗派遣鄭覃為宣諭使，王璠為副，命他們向成德軍宣布朝廷的命令並賜錢一百萬貫，並赦囚徒，給孤獨、廢疾不能自立者粟帛各有差。鄭覃向成德軍諭以大義，士兵們也終于讓王承元去赴任了。同月，考功員外郎李渤上奏重提鄭覃等諫畋游事，考其為上下，但穆宗不回複，馮宿領考功後廢李渤之議。

長慶元年（821年），翰林學士元稹、李德裕與中書舍人李宗閔發生爭鬥。李宗閔的下級盟友右補闕楊汝士和禮部侍郎錢徽負責科舉，前宰相西川節度使段文昌和翰林學士李紳秘密將與自己交好的一些進士寫下來請求錢徽照顧。但發榜時，這些進士並未上榜，鄭覃的弟弟鄭朗、前宰相節度使裴度的兒子裴譔、李宗閔的女婿蘇巢、楊汝士的弟弟楊殷士卻都及第。這個結果引起了騷動，段文昌彈劾楊、錢不公，所取進士都無才藝，全憑關節。當穆宗諮詢翰林學士們的意見時，李德裕、元稹、李紳都贊成段文昌。穆宗命李宗閔的同僚王起安排重考，將錢徽、李宗閔、楊汝士貶為地方官，還取消了楊、錢選拔的鄭朗等10個考生的成績。一說這就是歷時大約40年的牛李黨爭的開端，黨爭的一方為李德裕，即李黨，另一方為李宗閔及其盟友牛僧孺等，即牛黨。鄭覃被捲入此次事件後，通常被認為是李黨。十一月，鄭覃以中大夫、行諫議大夫、雲騎尉、滎陽縣開國男食邑三百戶轉給事中，散官、勛如故。

二年（822年）五月，兵部尚書李逢吉指使李賞告發元稹的朋友和王府司馬于方受宰相元稹指使結交刺客欲刺殺裴度，左神策軍中尉上報此事。詔鄭覃與左僕射韓皋、李逢吉三人訊問于方，查無謀刺之事，但元稹和裴度也因此都被罷相。

唐敬宗年間

長慶四年（824年）正月，穆宗駕崩，唐敬宗繼位，鄭覃被任為御史中丞，十月又權知工部侍郎。寶曆元年（825年）閏七月，他又被任為京兆尹。九月，讚揚了高陵令劉仁師的德政。

唐文宗年間

拜相前

寶曆二年（826年）末，唐敬宗被弒，其弟唐文宗繼位，鄭覃被任為左散騎常侍。太和三年（829年），以本官充翰林侍講學士，因經籍淪喪，他考察古事，屢次為此進言，文宗因而詔令秘閣搜訪遺文，每日添寫，複為四庫書分藏于十二庫。時李德裕先後遭李逢吉和李宗閔排擠，鄭覃在禁中為其美言，文宗才仍保有複用他的心思。四年（830年）三月，他又任工部尚書。鄭覃對中國古典典籍很有研究，行事也很正直，文宗因此尊敬他。在鄭覃建議下，文宗准其所奏置五經博士，開始了一項工程，將修改、加注後的典籍刻在石碣上，在太學展覽，仿東漢蔡邕故事。這項工程直到開成二年（837年）十月鄭覃拜相時才完成，鄭覃進《石壁九經》一百六十卷。

五年（831年），李宗閔、牛僧孺拜相，由于鄭覃和李德裕交好，遭到李宗閔的薄待，鄭覃也因而嫉恨朋黨。李宗閔尤其對鄭覃僅僅作為翰林侍講學士卻能接近皇帝言事而感到不快，並因而推薦鄭覃僅任工部尚書而免去翰林侍講學士。但文宗喜歡研究典籍，想念鄭覃，在六年（832年）二月又任他為翰林侍講學士。七年（833年）春，李德裕代牛僧孺為相，這時文宗讚揚殷侑對典籍的了解，並與鄭覃相比較。李宗閔說：「也許鄭覃、殷侑對典籍都很有見解，但他們的意見不足採信。」李德裕則說：「鄭覃、殷侑的意見被別人忽略了，陛下可沒忽略。」六月，文宗沒有諮詢李宗閔就任命鄭覃為御史大夫，又不久，李宗閔被遣出京城任山南西道（軍部在今漢中）節度使。鄭覃曾受命與時任尚書右僕射李逢吉疏理刑獄。

八年（834年）九月，李德裕罷相，李宗閔複相，與文宗近臣李訓、鄭注聯合把李黨從重要崗位上驅逐下來。十月，鄭覃被任命為戶部尚書。同月，九年（835年）五月，鄭覃被貶授秘書監。六月，李宗閔的盟友京兆尹楊虞卿得罪，李宗閔出言相救，文宗怒斥：「你曾說鄭覃是妖氣，現在做妖的是鄭覃還是你？」李宗閔和楊虞卿被貶，八月，鄭覃又任刑部尚書，十月，任尚書右僕射，十一月兼判國子祭酒事。不久，以連州刺史楊敬之代鄭覃為國子祭酒。

李訓為了收攬人心，將鄭覃等有聲望卻屢被打壓的老臣引居高位。十一月，文宗、李訓、鄭注清洗當權宦官的計劃（甘露之變）失敗了。李訓、鄭注和很多朝臣如宰相王涯、賈餗、舒元輿都被宦官逮捕或殺害。在宦官脅迫下，文宗召鄭覃和左僕射令狐楚及兵部尚書王源中、吏部侍郎李虞仲等進宮複查王涯的供詞——王涯因受刑不過，供認他和其他官員意圖推翻文宗，讓鄭注當皇帝。令狐楚和鄭覃証實供詞是王涯所寫。隨後，文宗將兩人留在中書省參與機務。文宗下詔恤災。李訓、王涯、賈餗、舒元輿都被處死，鄭注被襲殺。不久，令狐楚起草詔書譴責王涯等宰相謀反，但語言空洞，暗示謀反之說不可信，觸怒了仇士良，因而未能拜相。在擔任侍講時屢次對文宗言及厚風俗、黜朋比的鄭覃則和李石一同被授予同中書門下平章事，相當于宰相。鄭覃當時的官爵為銀青光祿大夫、守尚書右僕射、上柱國、滎陽郡開國男食邑二千戶，拜相後又封滎陽郡開國公。

甘露之變當日令狐楚和鄭覃在查案之餘還在內殿談及榷茶事。

拜相

鄭覃和李石拜相後，以仇士良為首的宦官經常引用李訓和鄭注的事例羞辱朝臣。鄭覃、李石以李訓、鄭注最初是由宦官王守澄所推薦來回應。此後，朝臣就依靠鄭覃、李石來保証人身安全。十二月，鄭覃和李石力主不再牽連李訓等被誅官員的親屬。時任京兆尹張仲方不能匡正禁軍橫行幹政，鄭覃用薛元賞為京兆尹，出張仲方為華州刺史。文宗說宰相之任在選賢任能，李石說自己與鄭覃常以此事為切。文宗曾看著鄭覃說：「鄭覃你老了，當無妄，試諭我堪比漢朝何等君主？」鄭覃說：「陛下是漢文帝、漢宣帝一類君主。」文宗說：「豈敢指望這樣！」李石欲強文宗之志使其不怠，說：「陛下之問和鄭覃之對臣皆以為非。顏回匹夫罷了，自比于舜。陛下有四海，春秋富，當觀得失于前，日引月長，以與堯、舜齊名，奈何比漢文帝、漢宣帝而又自以為不及。只希望陛下開肆厥志，不以漢文帝、漢宣帝自安，則大業可成。」當月，揚言要屠殺朝臣的宦官鹽州巡邊使田全操回京，引發了長安的恐慌，中書省吏卒也有逃去。鄭覃建議李石避難，李石拒絕了，指出如果宰相逃跑了，整個長安都會陷入恐慌，如果災難真的要降臨，誰也跑不了。鄭覃同意了。

但宦官仍然掌控政權，朝臣惶惶不可終日。開成元年（836年），文宗改元大赦，在紫宸殿問對。鄭覃說近年赦令皆不及此。三月，昭義軍（軍部在今山西長治）節度使劉從諫言辭激烈地上疏為王涯、賈餗辯白，彈劾仇士良的罪行，仇士良等宦官才有所收斂，文宗和鄭覃、李石也得以行使權力。後來文宗又在紫宸殿論政，問及治亂之別是人之正邪還是時運所致，鄭覃認為由人，李石則認為也由時運。

文宗好文，時常向鄭覃和時任皇太子侍讀王起問政。鄭覃雖然博于經籍要義，卻不善著文，不喜歡進士浮華，甚至在開成初年奏禮部貢院罷進士科。春季，文宗看了中舉名單後對侍臣說：「以前文格不妥，此次進士題目是朕出的，結果似乎勝過去年。」鄭覃說：「陛下改詩賦格調，以正頹俗，但高鍇（時任中書舍人，權知禮部貢舉）也能勵精選士，仰副聖旨。」文宗在紫宸殿問對談及選士，鄭覃說：「南北朝多用善作文之士，所以不治。士有才堪用即可用，何必文辭？」文宗說：「進士及第且已曾為州縣官的，藩鎮奏署就准奏，其餘否。」鄭覃說：「此科多是輕薄之人，不必盡用。」文宗說：「輕薄敦厚都是有的，未必獨在進士。此科置了已二百年，也不可遽改。」鄭覃說：「亦不可過于尊崇。」

文宗曾對宰相們說：「百司弛慢，要重條舉。」于是指香爐說：「此爐一開始也華美完好，用之已久，沒有光彩。若不加飾，如何複初？」鄭覃對道：「丕變風俗，當考實效。自三十年已來，多不務實，取于顏情。如嵇、阮之流，不攝職事。」李石說：「這本是因為治平，人人無事，安逸所致。今之人俗也慕王夷甫，恥不能及之。」文宗說：「卿等輔朕，在振舉法度而已。」

開成元年前宰相李固言複相，和鄭覃、李石共事，他與李宗閔、楊嗣複交好，鄭覃憎惡他。

文宗很懷念李訓，每每對李石、鄭覃、李固言等稱李訓之才：「李訓倫常不如公等，但是天下奇才，公等不及。」

同時，在鄭覃推薦下，由起居郎周墀、水部員外郎崔球、監察御史張次宗、禮部員外郎孔溫業、國子祭酒高重等負責校定《九經》文字，不久又奉令刻上石頭。完工後，鄭覃加門下侍郎、弘文館學士、監修國史。四月，文宗曾在延英殿論及古詩好壞，鄭覃說：「孔子所刪的是三百篇。降此五言七言，辭非雅正，不足帝王賞詠。《詩》的《雅》《頌》都是下刺上所為，非上化下而作。王者採詩以考風俗得失，仲尼刪定以為世規。近代陳後主、隋煬帝都能作章句，卻不知王者大端，終有末年亡國。章句小道，願陛下不取。」當時起居郎有官缺，李固言薦周敬複、崔球、張次宗三人，鄭覃再三以為不可，說：「崔球游于宗閔之門，且赤墀下秉筆是千古法，不可朋黨。如果用裴中孺、李讓夷，臣不敢有絲毫異議。」文宗說：「公事不要有異議！」鄭覃說：「如果宰相的意見都相同，必有欺陛下的事！」李讓夷雖然于次年拜中書舍人，但也因鄭覃的言論而被李玨和楊嗣複厭惡，在文宗一朝不至達官。當月，鄭覃以李紳為河南尹。鄭覃又奏太學置五經博士各一人，又因這些博士沒有職田，五月，又奏請依王府官例給他們賜祿粟，獲准。同月，張仲方入為秘書監。外界議論認為鄭覃黨附李德裕，排擠張仲方。鄭覃擔心捲入朋黨，趁紫宸殿奏事時說：「丞郎缺人，臣欲用張仲方。」文宗說：「中台侍郎是朝廷的顯貴職位。張仲方作為牧守無政績，怎能做丞郎？」

九月，李石提出為前宰相宋申錫平反。宋申錫先前因被王守澄、鄭注誣告謀反而被貶。鄭覃、李固言都贊成，文宗也追複了宋申錫的生前官爵。太僕卿段伯倫去世，李石請文宗加其賻襚，鄭覃說：「自古殺身利社稷，未有如秀實（指段伯倫父段秀實）者。」文宗惻然，為之罷朝一日，加段伯倫賻贈。文宗常言：「順宗事不詳實，史臣韓愈莫非當時屈于人？昔漢朝司馬遷《與任安書》，辭多怨懟，故《武帝本紀》多失實。」鄭覃說：「武帝中年大發兵事邊，生人耗瘁，府庫殫竭，司馬遷所述沒有言過其實。」李石說：「鄭覃所陳是以武帝相諫，欲陛下終究盛德。」文宗說：「誠如此。靡不有初，鮮克有終。」鄭覃說：「陛下樂觀書，然要義不過一二，陛下所說是對的。宜時刻秉承。」

二年（837年）二月，李固言彈劾鄧州刺史王堪、隋州刺史鄭襄行為尤其不端，而王堪是鄭覃所舉，鄭覃認為李固言冒犯自己，說：「臣知堪，故用為刺史。舉天下不職的豈止這二人？」

三年（838年）正月，文宗命起居舍人魏謨獻其祖上魏徵之笏，鄭覃說：「在人不在笏。」文宗說：「你沒會我意，這也是甘棠之比。」這時李石、李固言都已不是宰相，和鄭覃共事的是陳夷行、楊嗣複、李玨。楊嗣複與鄭覃尤其有矛盾，李固言、李玨介入更激化紛爭。楊嗣複數次推薦進士杜勝之材堪任諫官，鄭覃不用。二月，鄭覃進位太子太師。不久，宰相間就發生了爭執，鄭、陳為一派（都被認為是李黨），楊、李為一派（都被認為是牛黨）。當月楊嗣複提請把被貶為衡州（今湖南衡陽）司戶參軍事的李宗閔升官並調近京城，因為怕鄭覃反對，托宦官秘密進言。文宗同意，在紫宸殿說李宗閔在外四五年，宜別授一官。鄭覃堅決反對，說如果文宗憐李宗閔處于遠地，移近內地三五百里即可，不可再用這等奸邪，如果李宗閔升官，他就辭職。楊、鄭在文宗面前發生了激烈的爭吵。文宗又想以李宗閔為一州刺史，鄭覃又認為太過，只同意以為洪州司馬、別駕，又說楊嗣複與李宗閔結黨包庇之，李宗閔之惡甚于唐玄宗時期奸相李林甫。楊嗣複說鄭覃之語大錯，李宗閔在位時無李林甫妒賢害能破人家族之事，並舉殷侑為韓益求官時，自己因韓益前年犯贓未准奏，而鄭覃卻找自己請託一事，指鄭覃才是結黨包庇之人。結果是李宗閔被任為杭州刺史。從此，朝廷的重要決策都蒙上了黨爭的色彩，文宗也難以駕馭。鄭覃因是太子太師，想從尚書省奏事，太子詹事馮定奏道：「據《六典》，太師居詹事府，不合于都省禮上。」文宗詔鄭覃于本司奏事，時人推美馮定。四月，奉敕每月與賓、詹左右春坊五品以上官員謁見皇太子，且每月二十六日、二十一日詣崇明門謁見，若遇陰雨休假即取以次雙日。文宗因旱災釋放獄中囚徒，出宮人劉好奴等五百餘人送到兩街寺觀，任其投奔親戚。在紫宸殿問對時，李玨舉晉武帝平吳後廣選宮女的反例賀文宗此舉德邁千古，鄭覃也說：「晉武帝因採擇之失，中原化為左衽；陛下以為殷鑑，放去是非常適合的。」文宗命鄭覃親往疏理京師刑獄，分命宰相祈雨於太廟太社白帝壇。八月，右拾遺竇洵直當衙論奏，鄭覃、楊嗣複認為這是瑣事，說竇洵直近乎求名。當月，宰相們在紫宸殿奏事，提出獎賞閒居蘇州累年新近上疏論兵的陸洿官職，李玨支持並舉竇洵直的例子，鄭覃說：「若苞藏則不可知。」並要文宗防朋黨。楊嗣複說：「鄭覃疑臣朋黨，乞陛下放臣歸去。」于是拜乞罷免。李玨說朋黨最近消亡了，鄭覃卻說近期又有小朋黨楊漢公、張又新、李續出現了。李玨又奏邊事來打斷鄭覃，鄭覃說：「論邊事安危，臣不如李玨；嫉朋黨之惡則李玨不如臣。」楊嗣複又以自己被鄭覃指為朋黨而奏請辭相，文宗慰勉之。當時文宗方以政事委楊嗣複，因此惡鄭覃直言。十月，皇太子李永薨，時鄭覃複為國子祭酒，作為冊使持節冊其謚號莊恪。十二月，鄭覃請辭。文宗沒有答應，罷其太子太師，允許他可以隔三五天才到中書省報到。

鄭覃清儉，陳夷行耿介，楊嗣複等很嫉恨他們。四年（839年）四月，又一場大爭執爆發了。文宗表揚度支杜悰的才幹，楊嗣複、李玨推薦他任戶部尚書。陳夷行說：「這應該由皇帝做決定，不能把權力都下放給臣僚。」李玨說：「陛下先前告訴我，皇帝應該選擇宰相，而不是懷疑宰相。」五月，文宗問延英殿政事時，陳夷行再次強調皇帝不能放權給臣僚。受到攻擊的李玨反駁：「陳夷行顯然是懷疑有宰相在從陛下手中盜取權力。我此前已多次請求退休，如果我能被任命為一位親王的老師，就有幸了。」鄭覃說：「陛下在開成元年和二年（指836年和837年）做得很好，開成三年和四年就稍差了。」楊嗣複說：「頭兩年，是鄭覃和陳夷行當宰相。開成三年和四年，臣和李玨也當了宰相。顯然，這是我的罪過。」又說：「我不敢再進宰相官署了！」便不顧文宗在場，顧自退場。文宗派一個宦官去安撫他，說鄭覃失言，鄭覃也做了部分道歉，說：「臣愚昧。我不是刻意針對楊嗣複，但他的反應表明他容不下我。」楊嗣複說：「鄭覃說政事一年不如一年，這不僅是怪罪臣，更是在指責陛下的聖德。」雖然文宗說鄭覃之言是偶然，此後，楊嗣複數日不入中書省，數次請辭。文宗正委任楊嗣複，于是五月，鄭覃、陳夷行都被罷相，鄭覃再次擔任右僕射，一作守尚書左僕射。鄭、陳都罷相後，楊嗣複才複提拔李宗閔為相。

嚴厚本改故宰相元載謚號為「忠」，鄭覃問：「元載有貪賄之罪，又放縱悍妻惡子，為什麼謚他『忠』？」嚴厚本說：「元載贓罪盈億是可惡，但當代宗朝有人不利于東宮時，元載有翊戴德宗之功。」于是元載得以改謚。

鄭覃與先前的宰相楊綰、鄭餘慶同為大儒，輔政期間也都議優學科，先經誼，黜進士，後文辭，但終無法將歷經安史之亂後的儒學恢復到貞觀、開元的水平。

唐武宗年間

五年（840年），唐文宗駕崩，其弟唐武宗繼位。不久，李德裕複相，想推薦鄭覃複相，鄭覃稱有足疾，不能勝任朝謁，拒絕了。于是授司空。會昌二年（842年），他以守司徒致仕，五月以太子太師致仕去世。

《舊唐書·楊假傳》載鄭覃任華州刺史時署楊假為從事，改浙西道觀察使後又以楊假為大理評事。鄭覃從未擔任華州刺史和浙西道觀察使，這是將鄭朗誤記作鄭覃所致。

鄭覃年輕時清苦貞退，不輕易與人款狎。位至宰相後，居所未嘗增飾，僅能遮蔽風雨。家無媵妾，人們都仰慕其清高。但嫉惡太過，容不下的事很多，眾人因而害怕厭惡他。

後唐明宗年間，流傳的諸經多有錯誤，宰相馮道、李愚委學官田敏等取長安的鄭覃所刊石經，雕為印版，流布天下，後進賴之。清朝嚴可均與丁溶同治石經，著校文十卷，自序稱「鄭覃、唐元度輩皆通儒，頗見古本。」並提到雖然後唐的雕版歷宋朝、元朝、明朝轉刻生誤，但石經原本尚在。

作品

• 與他人合撰《唐六譯金剛經贊》一卷

子孫

• 鄭裔綽，秘書監

　• 鄭氏，嫁山東舊門九品衛佐崔皋。文宗重其不與權臣通婚。

注釋及參考文獻

Background

It is not known when Zheng Tan was born. He came from a prominent line, as his father Zheng Xunyu was a chancellor during the reigns of Emperor Dezong and Emperor Dezong's son Emperor Shunzong. As a result of his father's status, Zheng Tan was able to start his official career as a copyeditor (校書郎, Xiaoshu Lang) at Hongwen Institute (弘文館). He subsequently went through the ranks of low-level advisory officials as Shiyi (拾遺) and then Bujue (補闕); he then served successively as Kaogong Yuanwailang (考功員外郎), a low-level official at the ministry of civil service affairs (吏部, Libu), and then Xingbu Langzhong (刑部郎中), a supervisory official at the ministry of justice (刑部, Xingbu).

During Emperor Xianzongs reign

In 819, during the reign of Emperor Shunzong's son Emperor Xianzong, Zheng Tan was made Jianyi Daifu (諫議大夫), a fairly high-level advisory official. On an occasion when Emperor Xianzong wanted to make five eunuchs directors of troop food supplies for the northwestern border (with Huigu and Tufan), Zheng submitted a petition against the action, and the order was cancelled.

During Emperor Muzongs reign

In 820, Emperor Xianzong died and was succeeded by his son Emperor Muzong. Emperor Muzong was considered pleasure-loving, and in winter 820, there was an occasion when Emperor Muzong met with the imperial officials in the palace. Zheng Tan, his colleague Cui Yan (崔郾), and three other advisory officials took the opportunity to advise him that he was spending too much time in feast and games, and was overly exhausting the imperial treasury with his rewards to his favorites. Emperor Muzong was much surprised and initially displeased by the advice, and asked the chancellor Xiao Mian who these officials were. Xiao informed him that these were advisory officials. Emperor Muzong thus understood that they were giving him advice, and he dismissed them with kind words, but did not actually carry out their advice.

Around that time, Wang Chengzong the military governor (Jiedushi) of Chengde Circuit (成德, headquartered in modern Shijiazhuang, Hebei) — whose circuit had long been resistant to imperial rule — died. In light of Wang Chengzong's death, the Chengde soldiers wanted to support his brother Wang Chengyuan as the new military governor, but Wang Chengyuan wished to turn control of the circuit over to the imperial government. Nevertheless, the soldiers were not allowing him to depart and head for his new post as the military governor of Yicheng Circuit (義成, headquartered in modern Anyang, Henan). Wang Chengyuan requested Emperor Muzong to send a senior official to encourage the soldiers to obey the imperial order, so Emperor Muzong sent Zheng to Chengde, assisted by Wang Fan (王璠), to declare the imperial orders and to give a large cash award to the Chengde soldiers. It was said that when Zheng arrived at Chengde, he explained to the soldiers the importance of loyalty to the imperial government, and the soldiers thus allowed Wang Chengyuan to leave.

In 821, Zheng was involved in an incident that was considered one of the precipitating incidents of the coming Niu-Li Factional Struggles. At that time, both Yuan Zhen, an official at the legislative bureau of government (中書省, Zhongshu Sheng) and imperial scholar Li Deyu were involved in power struggles with another official, Li Zongmin. Meanwhile, Li Zongmin's junior colleague at the legislative bureau, Yang Rushi and the deputy minister of rites (禮部侍郎, Libu Shilang) Qian Hui (錢徽) were in charge of overseeing the imperial examinations. The military governor Duan Wenchang (a former chancellor) and the imperial scholar Li Shen both made secret pleas to Qian for certain examinees. However, when the results were announced, the examinees that Duan and Li Shen recommended were not given passing results, while among those passing the examinations were Zheng Tan's brother Zheng Lang; Pei Zhuan (裴譔) the son of the military governor Pei Du (a former chancellor); Li Zongmin's son-in-law Su Chao (蘇巢); and Yang Rushi's brother Yang Yinshi. This thus brought a popular uproar, and Duan submitted a report accusing Yang Rushi and Qian of being unfair. When Emperor Muzong requested opinions from the imperial scholars, Li Deyu, Yuan, and Li Shen all agreed with Duan's opinion. Emperor Muzong thus ordered Li Zongmin's colleague Wang Qi (王起) to conduct a re-examination, while demoting Qian, Li Zongmin, and Yang Rushi to be prefectural prefects and deposing 10 of the examinees selected by Qian and Yang Rushi. This was said to be the start of some 40 years of struggles between Li Deyu and his associates (known as the Li Faction) and Li Zongmin and his associates (known as the Niu Faction, named after Li Zongmin's ally Niu Sengru). (Despite the way the incident occurred, Zheng himself was usually viewed as a leader of the Li faction as well.) Late in 821, Zheng Tan was made an imperial attendant (給事中, Jishizhong).

During Emperor Jingzongs reign

In 824, the same year that Emperor Muzong died and was succeeded by his son Emperor Jingzong, Zheng Tan was made deputy chief imperial censor (御史中丞, Yushi Zhongcheng). Late in the year, he was also made acting deputy minister of public works (工部侍郎, Gongbu Shilang). In 825, he was made the mayor of Jingzhao Municipality (京兆, i.e., the region of the capital Chang'an).

During Emperor Wenzongs reign

Before chancellorship

After Emperor Jingzong was assassinated around new year 827 and was succeeded by his brother Emperor Wenzong, Zheng Tan was made Zuo Sanqi Changshi (左散騎常侍), a senior advisory official at the examination bureau of government (門下省, Menxia Sheng). In 829, he was also given the title of assistant imperial scholar. In 830, he was made deputy minister of public works. It was said that Zheng was learned in the Confucian classics and was upright in his behavior, and therefore Emperor Wenzong respected him. At Zheng's suggestion, Emperor Wenzong started a project where the text of the Confucian classics were edited, commentaries were written, and the resulting text carved onto stone tablets and displayed publicly at the imperial university. (This project would not be completed until 837, when Zheng would be chancellor.)

As of 831, Li Zongmin and Niu Sengru were chancellors, and Li Zongmin, because Zheng was friendly with Li Deyu, viewed Zheng pejoratively. He was, in particular, disliking the fact that Zheng, as assistant imperial scholar, would have access to the emperor. He thus recommended Zheng to be the minister of public works (工部尚書, Gongbu Shangshu) but had him stripped of his assistant imperial scholar status. However, Emperor Wenzong, who was interested in studying the Confucian classics, missed Zheng, and he again made Zheng assistant imperial scholar in 832. In 833, after Li Deyu became chancellor to replace Niu, there was a time when Emperor Wenzong complimented the knowledge that Yin You (殷侑) had about the classics, comparing him to Zheng. Li Zongmin responded, "Perhaps Zheng Tan and Yin You have understanding of the classics, but their suggestions are not worth listening to." Li Deyu responded, "The suggestions of Zheng Tan and Yin You are ignored by others, but not by Your Imperial Majesty." Soon thereafter, without further consulting Li Zongmin, Emperor Wenzong made Zheng the chief imperial censor (御史大夫, Yushi Daifu). Soon thereafter, Li Zongmin was sent out of the capital to serve as the military governor of Shannan West Circuit (山南西道, headquartered in modern Hanzhong, Shaanxi).

In 834, Zheng was made the minister of census (戶部尚書, Hubu Shangshu). Later in the year, Li Deyu was stripped of his chancellor post, and Li Zongmin, who again became chancellor, worked in conjunction with Emperor Wenzong's close associates Li Xun and Zheng Zhu to eject Li Deyu's allies out of important posts. Zheng Tan was thereafter made Mishu Jian (秘書監), the head of the Palace Library. In summer 835, after Li Zongmin and his ally Yang Yuqing offended Emperor Wenzong and were exiled, Zheng was made the minister of justice (刑部尚書, Xingbu Shangshu). In winter 835, he was made You Pushe (右僕射), one of the heads of the executive bureau (尚書省, Shangshu Sheng) as well as the principal of the imperial university (國子祭酒, Guozi Jijiu).

Soon thereafter, a plot by Emperor Wenzong, Li Xun, and Zheng Zhu to slaughter the powerful eunuchs (later known as the Ganlu Incident) ended in failure — with the eunuchs arresting or slaughtering Li Xun, Zheng Zhu, and many other imperial officials, including Li Xun's fellow chancellors Wang Ya, Jia Su, and Shu Yuanyu — at the eunuchs' prompting, Emperor Wenzong summoned Zheng Tan and his Pushe colleague Linghu Chu to the palace to examine a confession by Wang — extracted under torture — that he and the other officials were set to overthrow Emperor Wenzong and replace him with Zheng Zhu. Linghu and Zheng Tan confirmed that the handwriting was Wang's. Subsequently, Emperor Wenzong kept Zheng Tan and Linghu at the Office of the Chancellors to oversee the aftermaths. (Li Xun, Wang, Jia, and Shu were subsequently executed, while Zheng Zhu was killed in an ambush.) Linghu, however, subsequently offended the leading eunuch Qiu Shiliang by drafting an edict that, while condemning Wang and the other chancellors of treason, was in such empty language to imply the lack of believability of the treason allegations, and was not made chancellor. Zheng Tan was subsequently made chancellor de facto with the designation Tong Zhongshu Menxia Pingzhangshi, along with Li Shi. Zheng was also created the Duke of Yingyang.

Chancellorship

At the time that Zheng Tan and Li Shi served as chancellors, the eunuchs, led by Qiu Shiliang, often tried to humiliate the imperial officials by citing the examples of Li Xun and Zheng Zhu. It was said that Li Shi and Zheng Tan responded by pointing out that Li Xun and Zheng Zhu had initially been recommended to Emperor Wenzong by the eunuch Wang Shoucheng. Thereafter, it was said that the eunuchs' arrogance subsided somewhat, and the imperial officials depended on Li Shi and Zheng Tan to protect them. Soon thereafter, there was an incident in which, when the eunuch Tian Quancao (田全操) returned to the capital, he had remarked that when he got to the capital he would slaughter the officials, causing a general panic in Chang'an. Zheng suggested to Li Shi that they find refuge when Tian was approaching Chang'an, but Li Shi refused, pointing out that if even the chancellors fled, the panic would overtake the capital entirely, and that if there were to be disaster, they would not be able to get away anyway. Zheng agreed. Subsequently, Zheng and Li Shi urged that the relatives of Li Xun and the other executed officials be no longer pursued.

However, the eunuchs continued to be in control of the government, and it was said that the imperial officials worried about being slaughtered on a daily basis. It was not until spring 836, when Liu Congjian the military governor of Zhaoyi Circuit (昭義, headquartered in modern Changzhi, Shanxi), submitted harshly worded petitions protesting the innocence of Wang Ya and Jia Su and accusing Qiu of crimes, did Qiu and the other eunuchs began to cut back on their behavior and allow Emperor Wenzong, as well as Li Shi and Zheng, to exercise their authority.

It was said that Zheng, while well-learned, was not a good writer, and he despised the Jinshi imperial examinees for their overly-grand writing. He therefore suggested that the Jinshi examinations be abolished — a suggestion that Emperor Wenzong did not accept. Still, when Emperor Wenzong and Zheng had a discussion about poetry, Zheng pointed out that the poetry of the time often had inflated language and often were completely not descriptive. Meanwhile, with the imperial university carrying out the classics-carving project, at Zheng's recommendation, the officials Zhou Chi, Cui Qiu (崔球), Zhang Cizong, and Wen Ye (溫業) were put in charge of the project. When the project was completed, Zheng was given the additional offices of Menxia Shilang (門下侍郎, the deputy head of the examination bureau) and imperial scholar at Hongwen Institute (弘文館), and was put in charge of editing the imperial history.

In 836, with Li Guyan, a former chancellor, serving again as chancellor with Zheng and Li Shi, Li Shi advocated the posthumous rehabilitation of the former chancellor Song Shenxi, whom Wang Shoucheng and Zheng Zhu had previously accused of treason, leading to Song's exile. Zheng and Li Guyan joined Li Shi in his petition, and Emperor Wenzong agreed, restoring Song's offices posthumously.

By 838, Li Shi and Li Guyan were no longer chancellor, and serving with Zheng were Chen Yixing, Yang Sifu, and Li Jue. Soon, the chancellors were frequently arguing with each other, with Zheng and Chen (both of whom were considered Li Faction leaders) on one side and Yang and Li Jue (both of whom were considered Niu Faction leaders) on the other. For example, when Yang advocated for Li Zongmin, who was then forced to serve in the lowly post of military advisor to the prefect of Hengzhou (modern Hengyang in Hunan), to be promoted closer to the capital, Emperor Wenzong agreed, but Zheng opposed vehemently, stating as far that he would resign his chancellorship if Li Zongmin were promoted, leading to a heated argument between Yang and Zheng in Emperor Wenzong's presence. (In the aftermaths of the argument, Li Zongmin was made the prefect of Hang Prefecture (杭州, in modern Hangzhou, Zhejiang).) It was said that from that point on, every important decision became colored by factional politics, such that Emperor Wenzong could not rule on them easily. Late in 838, Zheng offered to resign. In response, Emperor Wenzong, while not accepting the resignation, allowed him to report to the Office of the Chancellors only once every three to five days.

In 839, there was another major argument between the chancellors that led to Zheng's and Chen's removal. Emperor Wenzong had praised the talents of the acting director of finances, Du Cong. Yang and Li Jue thereafter recommended Du to be the ministry of census. Chen responded, "Such orders should come from the Emperor. In the past, those who lost stately sovereignty did so by losing their authorities to their subjects." Li Jue responded, "Your Imperial Majesty had told me previously that an Emperor should select chancellors, not suspect them." In a subsequent discussion, Chen again emphasized that the Emperor should not yield authority to his subjects. Li Jue, offended, responded, "Chen Yixing is obviously suspecting that there are chancellors who are stealing power from Your Imperial Majesty. I have often requested retirement, and I would be fortunate to be given a post as an imperial prince's teacher." Zheng then stated, "Your Imperial Majesty ruled well in the first and second years of the Kaicheng era 836 and 837), while less so in the third and fourth years of Kaicheng 838 and 839)." Yang responded, "In the first and second years, Zheng Tan and Chen Yixing were in power. In the third and fourth years, your subject and Li Jue joined them. Of course, the crime is mine." He then stated, "I do not dare to again enter the Office of the Chancellors!" He withdrew from Emperor Wenzong's presence. Emperor Wenzong subsequently sent a eunuch to comfort him, and Zheng partially apologized, stating, "Your subject is foolish. I did not intend to point at Yang Sifu, but Yang Sifu's reaction shows that he has no tolerance for me." Yang responded, "Zheng Tan stated that the governance is deteriorating year by year. This does not only incriminate your subject, but also speaks ill of your holy virtues." Yang then submitted multiple offers to resign. Soon thereafter, Zheng and Chen were stripped of their chancellor posts, and Zheng was again made You Pushe.

During Emperor Wuzongs reign

In 840, Emperor Wenzong died and was succeeded by his brother Emperor Wuzong. Soon thereafter, Li Deyu became chancellor and wanted to recommend Zheng Tan to serve as chancellor again, but Zheng declined, claiming a foot illness. In 842, he retired with the title of acting Situ (司徒, one of the Three Excellencies). He died that year.