（３）辨上下：《内经》论脉有三部九候之诊，论病机有上下之属，并有“气反者，病在上取之下，病在下取之上，病在中傍取之”（《素问·五常政大论》）的治法。仲景撰用《素问》，对病机进行简要概括，亦有“上焦得通”（２３０条），“理中者理中焦，此利在下焦”（１５９条），“以热在下焦”（１２４条）等等。揆其词义，是此处之三焦，则上焦主胸胁，中焦主大腹脾胃，下焦指少腹大肠等而言，当是纯主疾病之部位，与手少阳经三焦，义自有别。此外，治法用于补偏救弊，或因势利导，根据病机上下之属，自有重要实际意义。如“伤寒不大便六七日，头痛有热者，与承气汤”（５６条）。治法是采取“高者抑之”（见《素问·至真要大论》下同）。若“少阴病，下利，脉微涩，呕而汗出，必数更衣，反少者，当温其上，灸之”（３２５条），则是宗“下者举之”的治法。
表里上下，是指机体调节功能的升降出入而言，若生理功能失常，即是病态，表里是偏于横的方面，上下则多指竖的方面。然而上与表，下与里又有千丝万缕的紧密联系。如“伤寒先厥后发热，下利必自止，而反汗出，咽中痛者，其喉为痹。发热无汗，而利必自止，若不止，必便脓血，便脓血者，其喉不痹”（３３４条），是厥阴热利，阳回发热。若阳复太过，则热势向外向上，迫津外泄则汗出，上扰咽部则为喉痹；若热郁于里则发热无汗，伤及在下之阴络，则为下利不止，便脓血。此种病机变化互相影响的规律，当可供人们深入研究。
（４）辨虚实：外感热病整个发展的过程，就是邪正双方激烈斗争此胜彼衰的全过程。所谓邪，是指外感六淫之邪或因脏腑功能失调而产生的致病因素。所谓正，是指病人本身的正气及抗病能力强弱而言。《内经》谓“邪气盛则实，精气夺则虚”，故虚实二者，实为辨别邪正盛衰的纲领。
《伤寒论》在具体条文中，着重运用虚实理论而阐明病机变化。如“发汗后，恶寒者，虚故第三卷１７０也；不恶寒，但热者，实也，当和胃气，与调胃承气汤”（７０条）。说明同患表病，同在发汗之后，在阳盛之体，往往能伤津化燥，转化为燥热实证。若阳虚之体，则汗后促使表阳更虚，又易转化为虚寒之证。故此条前者属汗后阳虚恶寒，与“发汗病不解，反恶寒者，虚故也，芍药甘草附子汤主之”（６８条）的病机略同；其严重者，并可发展为阳衰阴盛厥冷四逆的四逆汤证。后者汗后燥热成实，与“太阳病三日，发汗不解，蒸蒸发热者，属胃也，调胃承气汤主之”（２４８条）同一类型，同为阳明胃家实之证。
后之释《伤寒论》者，运用虚实理论常结合六经之发病原因部位及属性而分。如前章所云太阳伤寒为表实，中风为表虚。少阴病有里虚寒证，又有里虚热证者是也。又如结胸，主证为心下、胸胁硬满疼痛，大便不通，脉沉紧有力。若伴见烦躁懊，舌上干燥而渴，则是痰水与热结于心下胸胁间，仲景特显著标名为“结胸热实”（１３５条）。若见结胸主证而“无热证者”，则是寒痰结于心下，成为实证，病名“寒实结胸”（１４１条）者是也。
《伤寒论》原文用于平脉辨证之中，有词句类似而其意义不同，甚至恰如反正之比者。例如“阳明之为病，胃家实是也”（１８０条）。此条即指阳明病机总的为胃燥热实，又当有阳明主要脉证之身大热、汗自出、不恶寒、反恶热、脉滑数洪大等而言。若“脾家实”，则是太阴受病，脾阳得振，转输功能恢复，寒湿浊邪得以下趋为顺。故曰：“虽暴烦下利日十余行，必自止。以脾家实，腐秽当去故也”（２７８条）。胃家实是阳明热实，病势多为发展阶段，为病进。脾家实为太阴寒证，病势已向好的方面转化，为病退。又有“脉沉实者，以下解之”（３９４条）。其脉当指沉实有力，其证多有腹满痛，大便不通，小便黄赤，潮热，舌苔黄燥等燥结热实之象。若“伤寒下利日十余行，脉反实者，死”（３６９条），则阴寒邪盛，正气消索，脉见刚劲不柔之象，似即《内经》所谓真脏脉见，不仅脉不应病而已，与一般实脉自有本质的区别，故直断为死证。
此外，又有虚证似实或实证似虚者。前者如少阴虚寒脉微下利而面赤，证为阴盛阳浮，下寒上热，病名戴阳，亦即“至虚有盛候”，故用白通汤以破阴寒，而回阳气。若“大实有羸状”，在阳明胃燥热实，病久应下失下，往往见之。
（５）辨寒热：寒热是辨别疾病性质的纲领。凡病势亢奋呈进行性的多为热证，病势衰减呈退行性的多为寒证。一般热证是病邪虽盛，而正气较为充实，若寒证则是邪盛正衰，抗病能力低下。故《内经》有“气实者，热也；气虚者，寒也”（《素问·刺志论》）及“其身多热者易已，多寒者难已”（《灵枢·论痛篇》）等说。《伤寒论》在六经的前提下，往往提出主证，结合寒热，以探讨疾病之本质，而决定治疗的措施。例如下利：“自利不渴者，属太阴，以其脏有寒故也，当温之，宜服四逆辈”（２７７条）。“下利欲饮水者，以有热故也，白头翁汤主之”（３７３条）。一属太阴寒利，治宜温里扶阳；一属厥阴热利，方用苦寒清热。二者治法不容或紊，若进一步充实论证，分清寒热，当宗《内经》“诸病水液，澄彻清冷，皆属于寒”；“诸呕吐酸，暴注下迫，皆属于热”（《素问·至真要大论》）之病机进行辨析，则寒利除不渴外，当有大便稀溏、小便清长、腹满时痛等脏寒之象。热利亦应有大便臭秽、小便黄赤、里急后重，或下脓血、渴欲饮水等热象，方足为据。又如同一少腹满，亦有寒热之辨：如“太阳病六七日，……脉微而沉，反不结胸，其人发狂者，以热在下焦，少腹当硬满，小便自利者，下血乃愈。……抵当汤主之”（１２４条）。“病者手足厥冷，言我不结胸，少腹满，按之痛者，此冷结在膀胱关元也”（３４０条）。前者为太阳邪热入于下焦，与血相结，而为蓄血，故呈现神志失常阳盛发狂之热象。后者为冷结证，当属厥阴阴寒范畴，故有手足厥冷之阴寒证象。二者都以不结胸为辨，因结胸重者，有“从心下至少腹硬满而痛不可近”第三卷１７１（１３７条）之故也。
寒热之见于脉者，一般热证多见滑数洪大之象，如“服桂枝汤，大汗出后，大烦渴不解，脉洪大者，白虎加人参汤主之”（３６条）。“阳明病，谵语，发潮热，脉滑而疾者，小承气汤主之”（２１４条）。盖白虎、承气，正以清下阳明里热。若寒证则见迟涩之象。如“脉浮而迟，表热里寒，下利清谷，四逆汤主之”（２２５条），以急温少阴脏寒。盖其治法，正如《内经》“寒者热之，热者寒之”。
亦即《本草经》“疗寒以热药，疗热以寒药”之义。因辨别疾病属性的寒热，实为中医入门下手诊病的第一功夫，不可忽视。
（６）辨真假：在疾病发生发展的过程中，一般病机属寒，则证候呈现为寒象；反之属热则为热象。亦有病程颇久，邪入较深，病情严重，外表所呈现的证候有与本质不一致者。因此，六经病证中又有“真寒假热”、“真热假寒”之辨。例如“少阴病，下利清谷，里寒外热，手足厥逆，脉微欲绝，身反不恶寒，其人面色赤……通脉四逆汤主之”（３１７条）。此条下利清谷，手足厥逆，脉微欲绝，是脾肾阳衰里寒危重之证。面色赤，身反不恶寒，则为外热。外热是假，里寒是真。里寒外热，实即阴盛格阳之象。故用通脉四逆汤以破沉重之阴寒，而回垂绝之阳气。再有“伤寒，脉滑而厥者，里有热，白虎汤主之”（３５０条）。此证厥冷似寒，但滑为阳脉。此条以脉括证，以方测证，当概括有口干舌燥，烦渴引饮，胸腹灼热，恶热，小便黄赤等热象在内，故云里有热，亦即“厥深者热亦深”（１３５条）之谓。热郁于里而不得外达，致使阴阳气不相顺接为厥，是里真热而外假寒，故用白虎汤辛甘寒以清里除热。
另外，在《伤寒论》中，有证为阴寒而见阳脉者，亦有阳热实证而见阴脉者。如“病人脉数，数为热，当消谷引食，而反吐者，此以发汗，令阳气微，膈气虚，脉乃数也。数为客热，不能消谷，以胃中虚冷，故吐也”（１２２条）。此是中虚胃寒，虚阳上扰，不能消谷而反作吐，故脉现阳燥而数之象。又“脉迟为寒”（３３３条），但有“阳明病，脉迟，虽汗出，不恶寒者，其身必重，短气，腹满而喘，有潮热者，此外欲解，可攻里也；手足然汗出者，此大便已硬也。大承气汤主之”（２０８条）。此病为阳明燥热成实。燥屎阻于肠道，热结于里，阻碍营卫气血流行者，故脉迟。然阳热实证脉迟，必按而有力，阴寒虚证脉数，必按之无力，此从脉体中又当加以细审者。
（７）辨类似：六经病中，辨相类疑似之证，篇幅最多。如前所举“病有发热恶寒者，发于阳；无热恶寒者，发于阴也”（７条）。是病有恶寒，而以证候之发热与否而辨阴阳也。“伤寒不大便六七日，头痛有热者，与承气汤。其小便清者，知不在里，仍在表也，当须发汗，……宜桂枝汤”（５６条）。是同具头痛发热不大便之证，而以小便清否有表里之分也。其他如虚实寒热亦然。
故有同属一经之病，因病机不同，则治法自有不同。例如，太阳病，发热汗出恶风脉缓者，名为中风。太阳病，或已发热，或未发热必恶寒体痛呕逆，无汗而喘，脉阴阳俱紧，名为伤寒。前者主用桂枝汤辛甘温以解肌祛风调和营卫。后者用麻黄汤辛温之剂，以开泄腠理，解表发汗。二者同属太阳表病，而病候有风寒之别，证候有自汗脉缓、无汗脉紧之异，则其治法不容或紊。故曰：“桂枝本为解肌，若其人脉浮紧，发热不出者，不可与之也。常须识此，勿令误也”（１６条）。
又如五苓与抵当两证，病位同在下焦少腹，后世均列为太阳腑证。但前者属蓄水证，病由表邪入腑，膀胱水蓄，阳气不得宣化。其主证有少腹里急，烦渴，或渴欲饮水，水入则吐，小便不利。
后者由太阳邪热随经入里，与血相结，而为蓄血。其主证有神志失常发狂或喜忘，少腹硬满疼痛。二者证情类似而实有不同。故蓄血证数条，均以小便自利为辨证眼目。再如“阳明病，口燥，但欲漱水，不欲咽者，此必衄”（２０２条）。盖阳明为多血多气之经，故阳明热证，有气分之热第三卷１７２者，亦有热在血分者。此为阳明热邪熏灼血分，因血属流质，津液尚能自润，故口燥但欲漱水，不欲咽；热灼血分，损伤阳络，则为衄血。是与阳明气分之热，病有口干舌燥，大烦渴不解者，自有不同。
亦有不属一经之病，但其证候相类，病情在疑似之间者。如太阳蓄水证有烦渴，小便不利，与胃热津干之口渴心烦，大相类似。但一属太阳蓄水，治宜通阳利水：一为胃热津伤，治须和胃生津。二者性质不同，故五苓证首先提出胃中干烦渴与蓄水证之辨，可以会意。另外，如“伤寒中风，有柴胡证，但见一证便是，不必悉具”（１０１条）。其少阳病，往来寒热，胸胁苦满，默默不欲饮食，心烦，喜呕等，皆为柴胡主证之一。故有“设胸满胁痛者，与小柴胡汤”（３７条）。“呕而发热者，柴胡汤证具”（１４９条）等互为补充之条，然有类似柴胡证而不可用和解法者。如得病六七日，脉迟浮弱，恶风寒，手足温，医二三下之，不能食，而胁下满痛，面目及身黄，颈项强，小便难者，与柴胡汤，后必下重；本渴而饮水呕者，柴胡汤不中与也，食谷者哕”（９８条）。此条上段病属太阴里虚而兼表证。因误下更伤脾气，使脾阳功能不得充分运化水谷精微，湿不下行，进而影响肝胆疏泄功能，郁而不达，故出现有面目身黄胁下满痛等变证。下段为中虚水寒相搏而呕。此两例均与柴胡证类似而实不同，故仲景特为提出，以资鉴别。此外，如“胸中实”（３２４条）可吐证：或有恶风发热汗出而类似表证，但以头不痛项不强为异；或有饮食入口即吐，心下温温欲吐，复不能吐，手足厥冷，脉弦迟等，类似少阴阳微，寒饮上逆。然此病属胸中实，自必有“胸中痞硬，气上冲咽喉不得息”（１６６条），或“心下满而烦，饥不能食”（３５５条）等主证确实可凭，故可因势利导，处以酸苦涌吐之法，而主用瓜蒂散。
（3）辨别病位的上下：《黄帝内经》论述脉诊有三部九候的方法，论述病机有属于上或下之分，并提出了“气机逆反的病症，病位在上可从下部治疗，病位在下可从上部治疗，病位在中可从旁侧治疗”（出自《素问·五常政大论》）的治疗原则。张仲景撰写《伤寒论》时参考了《素问》，对病机进行了简要概括，例如提到“上焦气机通畅”（第230条）、“理中汤是调理中焦的，此处的腹泻属于下焦病变”（第159条）、“因为热邪位于下焦”（第124条）等。推敲这些词句的含义，这里的三焦，上焦主要指胸胁部位，中焦主要指大腹部的脾胃，下焦指少腹部的大肠等，这纯粹是指疾病所在的部位，与手少阳三焦经的概念意义自有不同。此外，治疗方法用于纠正偏颇、补救弊端，或顺应病势加以引导，根据病机属于上或下的情况，具有重要的实际意义。例如，“患伤寒病六七天不大便，伴有头痛和发热，给予承气汤”（第56条）。这种治法采取的是“病势高亢的予以抑制”（见《素问·至真要大论》，下同）的原则。如果“患少阴病，腹泻，脉象微涩，呕吐且出汗，必然频繁如厕但排便量少，应当温补上部，并进行艾灸”（第325条），则是遵循“病势低陷的予以提升”的治疗法则。
表里上下，是指机体调节功能的升降出入而言。如果生理功能失常，就是病态。表里偏向于横向的方面，上下则多指纵向的方面。然而上与表、下与里又有千丝万缕的紧密联系。例如，“患伤寒病先出现四肢厥冷，后来发热，腹泻必然会自行停止，但如果反而出汗，咽喉疼痛，喉部就会发生痹阻。如果发热无汗，腹泻也必然会自行停止，如果不停止，必然会出现大便带脓血，出现大便带脓血的人，喉部就不会发生痹阻”（第334条），这是厥阴病的湿热利，阳气回复而发热。如果阳气回复太过，则热势向外向上，逼迫津液外泄则出汗，上扰咽部则为喉痹；如果热邪郁结在体内则发热无汗，伤及在下部的阴络，则为腹泻不止，大便带脓血。这种病机变化互相影响的规律，可供人们深入研究。
（4）辨别虚实：外感热病整个发展的过程，就是邪气与正气双方激烈斗争、此胜彼衰的全过程。所谓邪，是指外感六淫之邪或因脏腑功能失调而产生的致病因素。所谓正，是指病人本身的正气及抗病能力的强弱而言。《黄帝内经》说“邪气盛实则表现为实证，精气耗损则表现为虚证”，所以虚实二者，实际上是辨别邪正盛衰的纲领。
《伤寒论》在具体条文中，着重运用虚实理论来阐明病机的变化。例如，“发汗后，感到恶寒的，是因为虚的缘故；如果不恶寒，只发热，是实证，应当调和胃气，给予调胃承气汤”（第70条）。这说明同样患有表病，同在发汗之后，在阳气旺盛的体质，往往能损伤津液化为燥热，转化为燥热实证。如果阳气虚弱的体质，则发汗后促使表阳更加虚弱，又容易转化为虚寒之证。因此这一条前者属于发汗后阳虚恶寒，与“发汗后病未解除，反而恶寒的，是因为虚的缘故，芍药甘草附子汤主治”（第68条）的病机大致相同；其严重者，并可发展为阳气衰微、阴寒内盛导致四肢厥冷的四逆汤证。后者发汗后燥热成实，与“太阳病三天，发汗后病未解，身体蒸蒸发热的，属于胃家实，调胃承气汤主治”（第248条）属于同一类型，同为阳明胃家实的证候。
后世解释《伤寒论》的人，运用虚实理论时常结合六经的发病原因、部位及属性来区分。如前章所说太阳伤寒为表实，中风为表虚。少阴病有里虚寒证，又有里虚热证的情况。又如结胸证，主证为心下、胸胁硬满疼痛，大便不通，脉象沉紧有力。如果伴见烦躁懊恼，舌上干燥而口渴，则是痰水与热邪结聚在心下胸胁间，张仲景特别显著地标名为“结胸热实”（第135条）。如果见到结胸的主证而“没有热象的”，则是寒痰结聚于心下，成为实证，病名“寒实结胸”（第141条）。
《伤寒论》原文用于平脉辨证之中，有词句类似而其意义不同，甚至截然相反的情况。例如“阳明病的特征，是胃家实”（第180条）。此条即指阳明病机总体上是胃燥热实，同时也应有阳明主要脉证的身大热、汗自出、不恶寒、反恶热、脉滑数洪大等而言。如果“脾家实”，则是太阴经受病，脾阳得以振奋，转输功能恢复，寒湿浊邪得以下行排出为顺。所以说：“虽然突然烦躁腹泻每天十几次，必然会自行停止。因为脾家实，腐败秽浊之物应当排出的缘故”（第278条）。胃家实是阳明热实，病势多为发展阶段，属于病进。脾家实为太阴寒证，病势已向好的方面转化，属于病退。又有“脉象沉实的，用攻下法解除”（第394条）。其脉应当指沉实有力，其证多有腹满痛，大便不通，小便黄赤，潮热，舌苔黄燥等燥结热实的征象。如果“患伤寒病腹泻每天十几次，脉象反而实大的，预后不良”（第369条），则是阴寒邪气盛实，正气消亡枯竭，脉象呈现刚劲不柔之象，似乎即《黄帝内经》所谓真脏脉现，不仅脉不应病而已，与一般实脉自有本质的区别，所以直接断定为死证。
此外，又有虚证似实或实证似虚的情况。前者如少阴虚寒脉微细、腹泻而面色红赤，证候为阴盛阳浮，下寒上热，病名戴阳，也就是“极度的虚会有强盛的假象”，所以用白通汤来破除阴寒，挽回阳气。如果“极度实的病会有羸弱的表现”，在阳明胃燥热实，病程久应下而未下，往往会出现这种情况。
（5）辨别寒热：寒热是辨别疾病性质的纲领。凡病势亢奋呈进行性的多为热证，病势衰减呈退行性的多为寒证。一般热证是病邪虽盛，而正气较为充实；若寒证则是邪盛正衰，抗病能力低下。故《黄帝内经》有“气实者表现为热；气虚者表现为寒”（《素问·刺志论》）及“身体多热的容易痊愈，多寒的难以痊愈”（《灵枢·论痛篇》）等说法。《伤寒论》在六经的前提下，往往提出主证，结合寒热，以探讨疾病的本质，从而决定治疗的措施。例如下利：“自行腹泻且不口渴的，属于太阴，因为其脏腑有寒的缘故，应当温补，适宜服用四逆汤类方剂”（第277条）。“腹泻想喝水的，因为有热邪的缘故，白头翁汤主治”（第373条）。一属太阴寒利，治宜温里扶阳；一属厥阴热利，方用苦寒清热。二者的治法不容混淆，若进一步充实论证，分清寒热，应当依据《黄帝内经》“各种水液分泌物，清澈透明的都属于寒”；“各种呕吐酸水，急骤的泄泻和里急后重，都属于热”（《素问·至真要大论》）的病机进行辨析，则寒利除不渴外，应当有大便稀溏、小便清长、腹满时痛等脏寒的征象。热利也应当有大便臭秽、小便黄赤、里急后重，或下脓血、渴欲饮水等热象，才足以作为依据。又如同样的少腹胀满，也有寒热之分：如“太阳病六七天，……脉象微而沉，反而没有结胸，其人发狂的，因为热邪在下焦，少腹部应当硬满，小便自利的，下血后才能痊愈。……抵当汤主治”（第124条）。“患者手足厥冷，说没有结胸，少腹胀满，按压疼痛的，这是冷结在膀胱关元穴”（第340条）。前者为太阳邪热入于下焦，与血相结，形成蓄血，故呈现神志失常、阳盛发狂的热象。后者为冷结证，当属厥阴阴寒范畴，故有手足厥冷的阴寒证象。二者都以不结胸为鉴别点，因为严重的结胸，有“从心下至少腹硬满而痛不可触碰”（第137条）的缘故。
寒热反映在脉象上，一般热证多见滑数洪大的象，如“服用桂枝汤后，大汗出，大烦渴不解，脉象洪大的，白虎加人参汤主治”（第36条）。“阳明病，谵语，发潮热，脉象滑而疾的，小承气汤主治”（第214条）。盖白虎汤、承气汤，正是用来清下阳明里热的。若寒证则见迟涩的象。如“脉象浮而迟，表热里寒，下利完谷不化的，四逆汤主治”（第225条），以急温少阴脏寒。其治法，正如《黄帝内经》所说的“寒者热之，热者寒之”。亦即《神农本草经》“治疗寒症用热药，治疗热症用寒药”的意义。因为辨别疾病属性的寒热，实为中医入门下手诊病的第一功夫，不可忽视。
（6）辨别真假：在疾病发生发展的过程中，一般病机属寒，则证候呈现为寒象；反之属热则为热象。亦有病程颇久，邪入较深，病情严重，外表所呈现的证候有与本质不一致的情况。因此，六经病证中又有“真寒假热”、“真热假寒”的辨别。例如“少阴病，下利清谷，里寒外热，手足厥逆，脉微欲绝，身体反而不恶寒，其人面色红赤……通脉四逆汤主治”（第317条）。此条下利清谷，手足厥逆，脉微欲绝，是脾肾阳衰、里寒危重的证候。面色红赤，身体反而不恶寒，则为外热。外热是假，里寒是真。里寒外热，实即阴盛格阳的现象。故用通脉四逆汤来破除沉重的阴寒，挽回垂绝的阳气。再有“患伤寒病，脉象滑而四肢厥冷的，里有热邪，白虎汤主治”（第350条）。此证厥冷似寒，但滑为阳脉。此条以脉括证，以方测证，应当概括有口干舌燥，烦渴引饮，胸腹灼热，恶热，小便黄赤等热象在内，所以说里有热，也就是“厥冷越深，热邪越深”（第135条，注：此处原文引用可能有误，通常指厥深热亦深）的意思。热邪郁结在里而不得外达，致使阴阳之气不相顺接而发生厥逆，是里真热而外假寒，故用白虎汤辛甘寒以清里除热。
另外，在《伤寒论》中，有证候为阴寒而见阳脉的情况，亦有阳热实证而见阴脉的情况。如“病人脉数，数为热，应当消化食物食欲旺盛，反而呕吐的，这是因为发汗，令阳气微弱，膈气虚弱，脉才数的。数为客热，不能消化食物，因为胃中虚冷，所以呕吐”（第122条）。这是中焦虚寒，虚阳上扰，不能消化食物反而作吐，故脉现阳燥而数的象。又“脉迟为寒”（第333条），但有“阳明病，脉迟，虽然出汗，不恶寒的，身体必定沉重，短气，腹满而喘，有潮热的，这是表证将要解除，可以攻里了；手足濈然汗出的，这是大便已经硬结。大承气汤主治”（第208条）。此病为阳明燥热成实。燥屎阻于肠道，热结于里，阻碍营卫气血流行，故脉迟。然而阳热实证脉迟，必须按之有力；阴寒虚证脉数，必须按之无力，这是从脉体中又应当加以仔细审察的地方。
（7）辨别类似：六经病中，辨别相类疑似之证的篇幅最多。如前面提到的“病有发热恶寒的，发于阳经；无热恶寒的，发于阴经”（第7条）。是有恶寒的症状，而以证候是否有发热来辨别阴阳。“患伤寒病六七天不大便，头痛有发热的，给予承气汤。如果小便清的，知道病不在里，仍在表，应当发汗，……适宜桂枝汤”（第56条）。是同样具有头痛发热不大便的证候，而以小便清否来区分表里。其他如虚实寒热也是如此。
所以有同属一经的病，因病机不同，则治法自然不同。例如，太阳病，发热汗出恶风脉缓的，名为中风。太阳病，或者已发热，或未发热必恶寒体痛呕逆，无汗而喘，脉阴阳俱紧的，名为伤寒。前者主用桂枝汤辛甘温以解肌祛风调和营卫。后者用麻黄汤辛温之剂，以开泄腠理，解表发汗。二者同属太阳表病，而病候有风寒之别，证候有自汗脉缓、无汗脉紧之异，则其治法不容混淆。所以说：“桂枝汤本是解肌的，如果病人脉浮紧，发热不出汗的，不可给予它。常常需要记住这一点，不要弄错”（第16条）。
又如五苓散证与抵当汤证两证，病位同在下焦少腹，后世均列为太阳腑证。但前者属蓄水证，病由表邪入腑，膀胱水蓄，阳气不得宣化。其主证有少腹里急，烦渴，或渴欲饮水，水入则吐，小便不利。后者由太阳邪热随经入里，与血相结，形成蓄血。其主证有神志失常发狂或喜忘，少腹硬满疼痛。二者证情类似而实有不同。故蓄血证数条，均以小便自利为辨证的着眼点。再如“阳明病，口干燥，只想漱水不想吞咽的，这一定会流鼻血”（第202条）。盖阳明是多血多气的经脉，故阳明热证，有气分之热的，亦有热在血分的。此为阳明热邪熏灼血分，因血属流质，津液尚能自润，故口燥只想漱水不想吞咽；热灼血分，损伤阳络，则为鼻衄。这与阳明气分之热，病有口干舌燥，大烦渴不解的情况，自有不同。
也有不属一经的病，但其证候相类，病情在疑似之间的情况。如太阳蓄水证有烦渴，小便不利，与胃热津干之口渴心烦，大相类似。但一属太阳蓄水，治宜通阳利水；一为胃热津伤，治须和胃生津。二者性质不同，故五苓散证首先提出胃中干烦渴与蓄水证的辨别，可以领会意思。另外，如“患伤寒中风，有柴胡证，只要见到一个证候就可以，不必全部具备”（第101条）。其少阳病，往来寒热，胸胁苦满，默默不欲饮食，心烦，喜呕等，皆为柴胡汤主证之一。故有“假设胸满胁痛的，给予小柴胡汤”（第37条）。“呕吐而发热的，柴胡汤证具备”（第149条）等互为补充的条文，然而有类似柴胡证而不可用和解法的。如得病六七天，脉迟浮弱，恶风寒，手足温，医生二三度攻下，不能进食，而胁下满痛，面目及身黄，颈项强，小便难的，给予柴胡汤，后来必然下重坠感；本来口渴而饮水呕吐的，柴胡汤不可给予，进食会呃逆”（第98条）。此条上段病属太阴里虚而兼表证。因误下更伤脾气，使脾阳功能不得充分运化水谷精微，湿不下行，进而影响肝胆疏泄功能，郁而不达，故出现有面目身黄胁下满痛等变证。下段为中焦虚寒水饮相搏而呕吐。这两例均与柴胡证类似而实不同，故张仲景特别提出，以供鉴别。此外，如“胸中实”（第324条）可用吐法：或有恶风发热汗出而类似表证，但以头不痛项不强为不同；或有饮食入口即吐，心下温温欲吐，复不能吐，手足厥冷，脉弦迟等，类似少阴阳微，寒饮上逆。然而此病属胸中实，自必有“胸中痞硬，气上冲咽喉不得呼吸”（第166条），或“心下满而烦，饥不能食”（第355条）等主证确实可凭，故可因势利导，处以酸苦涌吐之法，而主用瓜蒂散。
(3) Distinguishing Upper and Lower Locations: The *Huangdi Neijing* (Yellow Emperor's Inner Canon) discusses pulse diagnosis with the method of three regions and nine positions, and discusses pathogenesis as belonging to either upper or lower locations. It also proposes the treatment principle that "for conditions where Qi is reversed, if the disease is in the upper, treat from below; if the disease is in the lower, treat from above; if the disease is in the middle, treat from the sides" (*Suwen: Chapter on the Great Administration of Five Ordinances*). Zhang Zhongjing, when compiling the *Shanghan Lun* (Treatise on Cold Pathogenic Diseases), referenced the *Suwen* and briefly summarized pathogenesis, mentioning such points as "the Upper Jiao becomes unobstructed" (Clause 230), "Lizhong Tang regulates the Middle Jiao; this diarrhea belongs to the Lower Jiao" (Clause 159), and "because heat is located in the Lower Jiao" (Clause 124). Examining the meaning of these terms, the San Jiao (Triple Burner) referred to here denotes anatomical locations: the Upper Jiao primarily refers to the chest and hypochondrium, the Middle Jiao primarily refers to the great abdomen involving the Spleen and Stomach, and the Lower Jiao refers to the lower abdomen including the Large Intestine. This purely indicates the location of the disease, which differs in meaning from the Hand-Shaoyang San Jiao Meridian. Furthermore, treatment methods are used to correct deviations and remedy deficiencies, or to guide pathology according to its momentum. Based on whether the pathogenesis belongs to the upper or lower, there is significant practical importance. For example, "In a cold damage condition with constipation for six or seven days, accompanied by headache and fever, administer Chengqi Tang" (Clause 56). This treatment adopts the principle of "suppressing what is high" (see *Suwen: Chapter on the Great Essentials of Truth*, same below). If "in Shaoyin disease, with diarrhea, faint and choppy pulse, vomiting and sweating, frequent urges to defecate but scant output, one should warm the upper part and apply moxibustion" (Clause 325), this follows the treatment method of "lifting what is sinking."
Upper/Lower and Exterior/Interior refer to the ascending, descending, exiting, and entering functions of the body's regulatory mechanisms. If physiological functions become abnormal, it constitutes a pathological state. Exterior/Interior pertains more to the horizontal aspect, while Upper/Lower mostly refers to the vertical aspect. However, Upper and Exterior, Lower and Interior have intricate connections. For instance, "In cold damage, if there is initial cold extremities followed by fever, diarrhea will stop spontaneously. But if sweating occurs instead, and there is throat pain, bi obstruction of the throat will occur. If there is fever without sweating, diarrhea will also stop spontaneously; if it does not stop, pus and blood will appear in the stool. Those with pus and blood in the stool will not have throat bi obstruction" (Clause 334). This describes Jueyin heat-dampness diarrhea with Yang recovery and fever. If Yang recovery is excessive, heat trends outward and upward, forcing fluids out as sweat, and disturbing the throat causing bi obstruction; if heat stagnates internally, there is fever without sweat, damaging the Yin collaterals in the lower part, leading to persistent diarrhea with pus and blood. These patterns of mutual influence in pathogenesis offer valuable subjects for in-depth study.
(4) Distinguishing Deficiency and Excess: The entire development process of external febrile diseases is a process of intense struggle between pathogenic factors and healthy Qi, characterized by the rise and fall of each. "Pathogenic factors" refer to the Six Excesses (Six Evils) contracted externally or pathogenic factors generated by dysfunction of Zang-Fu organs. "Healthy Qi" refers to the patient's own righteous Qi and the strength of their resistance. The *Neijing* states, "When pathogenic factors are exuberant, it is Excess; when essence and Qi are depleted, it is Deficiency." Thus, Deficiency and Excess serve as the general principles for distinguishing the prosperity and decline of pathogens versus healthy Qi.
The *Shanghan Lun* emphasizes the use of Deficiency and Excess theory to elucidate changes in pathogenesis in specific clauses. For example, "After sweating, if the patient feels aversion to cold, it is due to Deficiency; if there is no aversion to cold but only fever, it is Excess. One should harmonize the Stomach Qi and administer Tiaowei Chengqi Tang" (Clause 70). This illustrates that although both suffer from exterior disease and have undergone sweating, in a constitution with abundant Yang, fluids are often damaged leading to dryness and heat, transforming into a Dry-Heat Excess syndrome. In a constitution with deficient Yang, sweating further depletes exterior Yang, easily transforming into a Deficient-Cold syndrome. Therefore, the former case belongs to post-sweating Yang deficiency with aversion to cold, similar in pathogenesis to "after sweating, the disease persists with increased aversion to cold, due to Deficiency, treated with Shaoyao Gancao Fuzi Tang" (Clause 68); severe cases may develop into Si Ni Tang syndrome characterized by Yang collapse and Yin excess with cold extremities. The latter case involves post-sweating dry-heat becoming excess, belonging to the same category as "Taiyang disease for three days, after sweating the disease persists with steaming fever, indicating Stomach Excess, treated with Tiaowei Chengqi Tang" (Clause 248), both being Yangming Stomach Excess syndromes.
Later commentators on the *Shanghan Lun* often distinguish Deficiency and Excess by combining them with the causes, locations, and attributes of the Six Meridians. As mentioned previously, Taiyang Cold Damage is Exterior Excess, while Wind Strike is Exterior Deficiency. Shaoyin disease includes Interior Deficient-Cold and Interior Deficient-Heat syndromes. For example, Jie Xiong (Chest Binding) has main symptoms of hardness, fullness, and pain in the epigastrium and hypochondrium, constipation, and a deep, tight, forceful pulse. If accompanied by restlessness and oppression, dry tongue, and thirst, it indicates phlegm-fluids binding with heat in the epigastrium and hypochondrium. Zhang Zhongjing specifically labels this "Jie Xiong Heat-Excess" (Clause 135). If the main symptoms of Jie Xiong appear but "without heat signs," it indicates cold-phlegm binding in the epigastrium, forming an Excess syndrome named "Cold-Excess Jie Xiong" (Clause 141).
In the original text of the *Shanghan Lun*, used for pulse-taking and syndrome differentiation, there are phrases that look similar but have different meanings, or even opposite meanings. For example, "The definition of Yangming disease is Wei Jia Shi (Stomach Family Excess)" (Clause 180). This clause refers to the general Yangming pathogenesis of dry-heat-excess in the Stomach, and should be understood alongside major Yangming pulse and syndrome manifestations such as high fever, spontaneous sweating, aversion to cold absent but aversion to heat present, and slippery, rapid, large pulses. If "Pi Jia Shi (Spleen Family Excess)," this means the Taiyin organ is affected, Spleen Yang recovers, transport functions resume, and cold-damp turbid pathogens descend naturally. Thus it says: "Although sudden irritability and diarrhea occur ten-plus times a day, it will stop spontaneously. Because Pi Jia Shi, the rotting filth must exit" (Clause 278). Wei Jia Shi is Yangming Heat-Excess, with a disease trend developing forward, indicating progression. Pi Jia Shi is a Taiyin Cold syndrome, with a disease trend turning for the better, indicating regression. Also, "If the pulse is deep and excess, resolve it with purgation" (Clause 394). The pulse should be deep, excess, and forceful; symptoms likely include abdominal fullness and pain, constipation, yellow-red urine, tidal fever, and yellow-dry coating, indicating dry-binding heat-excess. However, "In cold damage with diarrhea ten-plus times a day, if the pulse is unexpectedly excess, it is fatal" (Clause 369). This indicates overwhelming Yin-cold pathogen and exhaustion of healthy Qi, with a pulse showing rigid and unyielding qualities, resembling the True Organ Pulse described in the *Neijing*. This is essentially different from a general excess pulse, hence directly judged as a fatal syndrome.
Furthermore, there are Deficiency syndromes resembling Excess, or Excess syndromes resembling Deficiency. The former, such as Shaoyin Deficient-Cold with faint pulse and diarrhea but red face, represents Yin excess floating Yang, lower cold and upper heat, named Dai Yang (Floating Yang), i.e., "Extreme Deficiency presents with Excessive appearances." Hence, Bai Tong Tang is used to break Yin-cold and restore Yang. The latter, "Extreme Excess presents with Withered appearances," often occurs in Yangming Stomach dry-heat-excess where prolonged illness required purgation but was neglected.
(5) Distinguishing Cold and Heat: Cold and Heat are the guiding principles for distinguishing the nature of diseases. Generally, disease trends that are hyperactive and progressive are Heat syndromes; those that are declining and regressive are Cold syndromes. In general Heat syndromes, pathogens are strong but healthy Qi is relatively sufficient; in Cold syndromes, pathogens are strong while healthy Qi is weak, with low resistance. Thus, the *Neijing* states, "Qi Excess manifests as Heat; Qi Deficiency manifests as Cold" (*Suwen: Chapter on Acupuncture Ambitions*) and "Those with much heat heal easily; those with much cold heal with difficulty" (*Ling Shu: Chapter on Pain*). Under the premise of the Six Meridians, the *Shanghan Lun* often proposes main syndromes combined with Cold/Heat to explore the essence of the disease and determine treatment measures. For example, regarding diarrhea: "Spontaneous diarrhea without thirst belongs to Taiyin, because the Zang organs have Cold; it should be warmed, suitable for Si Ni Tang category" (Clause 277). "Diarrhea with desire to drink water is due to Heat, treated with Bai Tou Weng Tang" (Clause 373). One is Taiyin Cold-diarrhea, treated by warming the interior and supporting Yang; the other is Jueyin Heat-diarrhea, treated with bitter-cold herbs to clear heat. Their treatments must not be confused. To further substantiate and distinguish Cold from Heat, one should follow the *Neijing*: "All bodily fluids that are clear and cold belong to Cold"; "All vomiting of sour fluid, sudden urgent diarrhea with tenesmus belong to Heat" (*Suwen: Chapter on the Great Essentials of Truth*). Thus, Cold-diarrhea, besides lack of thirst, should have signs of Zang-Cold like loose stools, clear-long urination, and intermittent abdominal pain. Heat-diarrhea should have foul-smelling stools, yellow-red urine, tenesmus, or passage of pus/blood, and thirst/desire to drink, providing sufficient evidence. Similarly, for lower abdominal fullness, Cold and Heat must be distinguished: "Taiyang disease for six or seven days... pulse faint and deep, contrary to Jie Xiong, the person goes mad because heat is in the Lower Jiao, the lower abdomen should be hard and full, urination is free, bleeding resolves it... Di Dang Tang treats" (Clause 124). "Patient has cold extremities, says no Jie Xiong, lower abdomen full, painful on pressure, this is Cold Binding at Bladder Guanyuan point" (Clause 340). The former is Taiyang pathogenic heat entering the Lower Jiao, binding with blood to form Blood Accumulation, presenting with mental aberrations and Yang-excess mania (Heat signs). The latter is Cold Binding, belonging to Jueyin Yin-Cold, presenting with cold extremities (Yin-Cold signs). Both are distinguished by the absence of Jie Xiong, because severe Jie Xiong involves "hardness, fullness, and pain from epigastrium to lower abdomen intolerable to touch" (Clause 137).
Regarding pulses reflecting Cold/Heat, Heat syndromes generally show slippery, rapid, large pulses. For example, "After taking Guizhi Tang, profuse sweating occurs, followed by severe irritability and unquenchable thirst, with large pulse, Bai Hu Jia Ren Shen Tang treats" (Clause 36). "Yangming disease, delirium, tidal fever, slippery and rapid pulse, Xiao Chengqi Tang treats" (Clause 214). Bai Hu and Chengqi formulas clear and purge Yangming interior heat. Cold syndromes show slow and choppy pulses. For example, "Pulse floating and slow, exterior heat interior cold, diarrhea with undigested food, Si Ni Tang treats" (Clause 225), urgently warming Shaoyin Zang-Cold. This treatment aligns with the *Neijing*: "Cold is treated with Heat; Heat is treated with Cold," which is also the meaning of the *Shennong Ben Cao Jing*: "Treat Cold with Hot herbs, treat Heat with Cold herbs." Distinguishing the Cold/Heat nature of disease is the first skill in TCM diagnosis and cannot be ignored.
(6) Distinguishing True and False: In the development of disease, generally if pathogenesis is Cold, symptoms present as Cold signs; if Heat, as Heat signs. However, in prolonged courses with deep pathogen invasion and severe conditions, external symptoms may contradict the internal essence. Therefore, among Six Meridian diseases, there is distinction between "True Cold False Heat" and "True Heat False Cold." For example, "Shaoyin disease, diarrhea with undigested food, interior cold exterior heat, cold extremities, pulse faint and nearly imperceptible, body contrary to aversion to cold, person has red face... Tong Mai Si Ni Tang treats" (Clause 317). Here, diarrhea with undigested food, cold extremities, and faint pulse indicate critical Spleen-Kidney Yang collapse and interior cold. Red face and lack of aversion to cold represent exterior heat. The exterior heat is false; the interior cold is true. Interior cold with exterior heat is actually Yin excess repelling Yang. Hence, Tong Mai Si Ni Tang is used to break heavy Yin-cold and restore near-extinct Yang. Again, "Cold damage, pulse slippery with cold extremities, interior has heat, Bai Hu Tang treats" (Clause 350). This condition shows cold extremities resembling Cold, but slippery is a Yang pulse. Using pulse to summarize syndrome and formula to test syndrome, it implies dry mouth/tongue, irritability/thirst, burning chest/abdomen, aversion to heat, yellow-red urine, etc. Thus, "interior has heat," meaning "the deeper the cold extremities, the deeper the heat" (Clause 135). Heat stagnating internally prevents outward reach, causing Yin-Yang Qi misconnection resulting in cold extremities. This is True Heat inside with False Cold outside, so Bai Hu Tang (acrid-sweet-cold) clears interior heat.
Additionally, in the *Shanghan Lun*, there are Yin-Cold syndromes showing Yang pulses, and Yang-Heat Excess syndromes showing Yin pulses. For example, "Patient has rapid pulse. Rapid indicates Heat, should digest food and increase appetite, but instead vomits. This is due to sweating, making Yang Qi slight and Diaphragm Qi deficient, so the pulse is rapid. Rapid indicates guest heat, cannot digest food, because Stomach is deficient and cold, hence vomiting" (Clause 122). This is Middle Jiao deficiency-Stomach Cold, with deficient Yang disturbing upward, unable to digest food, hence vomiting, showing a dry-rapid pulse. Also, "Slow pulse indicates Cold" (Clause 333), but "Yangming disease, slow pulse, although sweating, no aversion to cold, body heavy, shortness of breath, abdominal fullness and panting, tidal fever, this is exterior resolving, interior can be attacked; hands and feet perspiring continuously, this is hardened stool. Da Chengqi Tang treats" (Clause 208). This is Yangming dry-heat becoming excess. Dry feces blocking intestines and heat binding internally obstruct Ying-Wei and Blood-Qi flow, hence slow pulse. However, Yang-Heat Excess with slow pulse must be forceful upon pressure; Yin-Cold Deficiency with rapid pulse must be empty upon pressure. This requires careful examination of the pulse body.
(7) Distinguishing Similarities: Among Six Meridian diseases, distinguishing similar and suspected syndromes occupies the most space. As previously cited, "Diseases with fever and aversion to cold originate in Yang; those without fever and aversion to cold originate in Yin" (Clause 7). This distinguishes Yin/Yang based on the presence or absence of fever despite aversion to cold. "Cold damage with constipation for six or seven days, headache and fever, give Chengqi Tang. If urine is clear, know it is not in the interior, still in the exterior, should induce sweating... Gui Zhi Tang is suitable" (Clause 56). This distinguishes Exterior/Interior based on urine clarity despite identical headache, fever, and constipation. The same applies to Deficiency/Excess and Cold/Heat.
Thus, diseases belonging to the same meridian may have different treatments due to different pathogenesis. For example, Taiyang disease with fever, sweating, aversion to wind, and slow pulse is named Feng Zhong (Wind Strike). Taiyang disease with existing or non-existing fever, necessarily aversion to cold, body pain, vomiting, wheezing, no sweat, and tight pulse at both Cun and Chi is named Shang Han (Cold Damage). The former mainly uses Guizhi Tang (acrid-sweet-warm) to release muscle layer, dispel wind, and harmonize Ying-Wei. The latter uses Mahuang Tang (acrid-warm) to open pores, release exterior, and induce sweating. Both are Taiyang exterior diseases, but differ in pathogen (Wind vs. Cold) and syndrome (spontaneous sweat/slow pulse vs. no sweat/tight pulse), so treatments must not be confused. Thus it says: "Guizhi Tang is for releasing the muscle layer; if the pulse is floating-tight and fever without sweat, do not give it. Always remember this, do not err" (Clause 16).
Similarly, Wu Ling San syndrome and Di Dang Tang syndrome both involve the Lower Jiao lower abdomen, classified by later generations as Taiyang Fu-organ diseases. However, the former is Water Accumulation syndrome, caused by exterior pathogen entering the Fu organ, bladder water accumulation, and Yang Qi failing to transform. Main symptoms include lower abdominal urgency, irritability/thirst, or drinking water leading to vomiting, and difficult urination. The latter is caused by Taiyang pathogenic heat following the meridian inward, binding with blood to form Blood Accumulation. Main symptoms include mental aberrations (mania or forgetfulness), and lower abdominal hardness/fullness/pain. Although similar, they are fundamentally different. Therefore, for Blood Accumulation clauses, free urination is the key diagnostic indicator. Again, "Yangming disease, dry mouth, only wanting to rinse water without swallowing, this will definitely result in nosebleed" (Clause 202). Yangming is a meridian with abundant blood and Qi. Thus, Yangming Heat syndromes include Heat in the Qi level and Heat in the Blood level. This is Yangming heat harassing the Blood level. Since blood is fluid and fluids can self-moisten, the mouth is dry but one only wants to rinse, not swallow. Heat scorching the Blood level damages Yang collaterals, causing epistaxis. This differs from Yangming Qi-level Heat, which features dry mouth/tongue, severe irritability, and unquenchable thirst.
There are also diseases not belonging to a single meridian but having similar syndromes and ambiguous conditions. For example, Taiyang Water Accumulation syndrome has irritability/thirst and difficult urination, very similar to Stomach Heat damaging fluids causing thirst/irritability. But one is Taiyang Water Accumulation, treated by promoting Yang and promoting diuresis; the other is Stomach Heat damaging fluids, treated by harmonizing Stomach and generating fluids. Their natures differ. Thus, Wu Ling San clause first distinguishes Stomach dryness-thirst from Water Accumulation syndrome for understanding. Additionally, "Cold damage or wind strike, if there are Chai Hu syndrome signs, seeing one sign is sufficient, no need for all to be present" (Clause 101). Shaoyang disease symptoms like alternating chills/fever, hypochondriac fullness/discomfort, silent desire not to eat, irritability, and preference for vomiting are main Chai Hu signs. Thus, "Assuming chest fullness and hypochondriac pain, give Xiao Chai Hu Tang" (Clause 37); "Vomiting with fever, Chai Hu syndrome is complete" (Clause 149) supplement each other. However, there are cases resembling Chai Hu syndrome where harmonization is contraindicated. For example, after 6-7 days of illness, pulse slow-floating-weak, aversion to wind/cold, warm extremities, physician purged two or three times, inability to eat, hypochondriac fullness/pain, yellow face/eyes/body, stiff neck, difficult urination. Giving Chai Hu Tang will cause rectal heaviness; if originally thirsty and vomiting after drinking, Chai Hu Tang is contraindicated, eating causes hiccups (Clause 98). The upper part of this clause is Taiyin Interior Deficiency with exterior syndrome. Improper purging further damages Spleen Qi, preventing Spleen Yang from fully transforming grain essence, dampness fails to descend, affecting Liver/Gallbladder drainage, leading to jaundice and hypochondriac pain. The lower part is Middle Deficiency with Cold-Water clashing causing vomiting. Both resemble Chai Hu syndrome but are different, so Zhang Zhongjing highlights them for differentiation. Furthermore, "Xiong Zhong Shi (Solidness in Chest)" (Clause 324) can be treated with emesis: or with aversion to wind, fever, sweating resembling exterior syndrome, but differing by no headache/stiff neck; or with immediate vomiting upon eating, epigastric nausea unable to vomit, cold extremities, string-tight-slow pulse resembling Shaoyin Yang micro and Cold-Phlegm rising. However, this disease belongs to Solidness in Chest, necessarily having main symptoms like "epigastric痞硬, Qi rushing to throat preventing breathing" (Clause 166) or "epigastric fullness with irritability, hunger but unable to eat" (Clause 355) as reliable evidence. Thus, following the momentum, sour-bitter emetic method is used, mainly Geng Ti San.