四诊者，望闻问切也。望以辨色，闻以辨声，问以辨证，切以辨脉。盖人禀气血以生，气血不和而为病，
有诸内者，必形诸外，但病变多端，其脉其证皆有真假，差之毫厘，失之千里，故圣人立法，必以四端互相
参合，方无错误，如《素问·疏五过论》、《征四失论》，谆切告诫，而俗学不明此理，妄意揣度，虚言矜夸，
谓切脉即能知病，欺人自欺，其害实甚。又如《难经》言∶假令得肝脉，其外证善洁、面青、善怒，其内证脐
左有动气，按之牢若痛，其病四肢满闭，淋溲便难，转筋，有是者肝也，无是者非也。此言诊得弦急之脉为肝
脉，必有肝脏之证，如面青、善怒等，方为肝病，如无肝证，即非肝病，各脏皆然，是故一脉所主，病有多
端，一病所现，亦有多脉，必当脉证互相印合，方能知其病由，故四诊之道，缺一不可，经文已详悉矣。惟辨
舌苔之法，未曾论及，止有《灵枢·师传篇》云∶
脾者主为卫，使之迎粮，视唇舌好恶，以知吉凶；《热病篇》言∶肺热病，舌上黄而已。盖脾主肌肉，卫者，
肌肉也，唇舌为肌肉之本，故视之可验病之吉凶也。若夫强弱寿夭之形，及一切病状，尤当于禀赋疾病诸门合
参，以明其理。本门最详者脉也，脉为气血流行之象，而有升降出入，故当与营卫经络、阴阳脏腑诸门同观。
必得悟其神理，指下方能明其为和为病、为虚、为实。古人论脉云∶吾心之所解，口莫能宣。此神理必须心悟，
非言语所能传。能明其脉，则证之真假自见；能辨其证，则脉之虚实自明。所谓真者，脉证相合，虚实显然也；
所谓假者，脉证不合，实病似虚、虚病似实也。真者易辨，假者难辨，不辨其假而误治之，杀人于反掌间，
可不畏哉！可不慎哉！
四诊，即望、闻、问、切。通过观察神色来辨别病情，通过听声音和嗅气味来辨别病状，通过询问病史来辨别证候，通过切按脉搏来辨别脉象。因为人依靠气血而生存，气血不和则导致疾病，内部有病变，必然在外部表现出来。但是病变情况复杂多变，其脉象和证候都有真假之分，如果差错毫厘，结果就会谬以千里。所以圣人制定法则，必须让这四种诊断方法互相参照印证，才能避免错误。如《素问·疏五过论》和《征四失论》中，反复恳切地告诫世人，但世俗的医者不明白这个道理，妄加猜测，用虚假言语自我夸耀，认为仅凭切脉就能知晓病情，这是欺人又自欺，危害极大。
又如《难经》中说：假如诊得肝脉（肝脏的脉象），其外在证候表现为爱干净、面色发青、容易发怒；其内在证候表现为肚脐左侧有搏动感，按压时坚硬且疼痛；其病症表现为四肢胀满闭塞、小便淋漓或困难、大便艰难、筋脉拘挛抽搐。具备这些症状的是肝病，不具备这些症状的就不是肝病。这里说的是诊得弦急之脉为肝脉，必须同时伴有肝脏的证候，如面色青、易怒等，才确认为肝病；如果没有肝脏的证候，就不是肝病。其他脏腑的情况也都如此。因此，一种脉象可能主多种疾病，一种疾病也可能出现多种脉象，必须将脉象与证候互相印证，才能了解疾病的根源。所以四诊的方法，缺一不可，经文已经阐述得很详细了。
唯独辨别舌苔的方法，文中未曾详细论述，只有《灵枢·师传篇》说：脾脏主要负责护卫身体，使其迎接水谷精微，通过观察嘴唇和舌头的好坏，来预知病情的吉凶；《热病篇》说：肺热病，舌苔发黄。这是因为脾主肌肉，卫气行于肌肉之中，嘴唇和舌头是肌肉的根本，所以通过观察它们可以验证病情的吉凶。至于体质的强弱、寿命的长短以及一切病状，尤其应当结合禀赋（先天体质）和疾病各门类进行综合参考，以阐明其中的道理。本门中最详尽的是脉诊，脉象是气血运行的表现，具有升降出入的变化，所以应当与营卫（营气与卫气）、经络、阴阳、脏腑等各门类知识结合起来观察。必须领悟其中的神妙原理，手指下才能明确判断是和缓还是病态、是虚证还是实证。古人论脉时说：我心里所理解的，嘴上无法完全表达出来。这种神妙的原理必须用心领悟，不是言语所能传授的。如果能明白脉象，那么证候的真假自然显现；如果能辨别证候，那么脉象的虚实自然明了。所谓“真”，是指脉象与证候相符合，虚实的性质非常明显；所谓“假”，是指脉象与证候不相符合，实证表现出虚象，虚证表现出实象。真实的证候容易辨别，虚假的证候难以辨别，如果不辨别虚假而误治，杀人只在眨眼之间，能不敬畏吗？能不谨慎吗！
The Four Diagnostic Methods refer to inspection (Wang), auscultation and olfaction (Wen), inquiry (Wen), and palpation (Qie). Inspection is used to distinguish colors, auscultation and olfaction are used to distinguish sounds and smells, inquiry is used to distinguish syndromes, and palpation is used to distinguish pulses. Since humans are born with Qi and Blood, disease arises when Qi and Blood are disharmonious. Internal pathology must manifest externally. However, pathological changes are manifold, and both pulses and syndromes may present as true or false. A discrepancy of a hair's breadth can lead to an error of a thousand miles. Therefore, the Sages established laws requiring the mutual cross-reference of these four methods to avoid errors. As repeatedly and earnestly warned in *Suwen: Shu Wu Gu Lun* (Plain Questions: Discourse on Avoiding Five Negligences) and *Suwen: Zheng Si Shi Lun* (Plain Questions: Discourse on Identifying Four Failures), lay practitioners who fail to understand this principle often speculate rashly, boast with empty words, and claim that pulse diagnosis alone suffices to know the disease. This deceives others and oneself, causing great harm.
Furthermore, *Nan Jing* states: Suppose one obtains the Liver Pulse. Its external manifestations include a preference for cleanliness, a bluish complexion, and irritability. Its internal manifestations include pulsating Qi at the left side of the umbilicus, which feels firm and painful upon pressure. Its diseases include fullness and obstruction of the four limbs, dribbling or difficult urination, difficult defecation, and tendon spasms. If these symptoms are present, it is a Liver disease; if absent, it is not. This indicates that obtaining a wiry and rapid pulse identified as the Liver Pulse requires accompanying Liver organ syndromes, such as a bluish complexion and irritability, to confirm Liver disease. Without Liver syndromes, it is not Liver disease. The same applies to other Zang organs. Thus, one pulse type may correspond to multiple diseases, and one disease may present with multiple pulses. One must mutually verify pulses and syndromes to ascertain the origin of the disease. Hence, the Four Diagnostic Methods are indispensable, as detailed in the classics.
However, the method of distinguishing tongue coating is not extensively discussed here. Only *Lingshu: Shichuan Pian* (Spiritual Pivot: Chapter on Teacher's Transmission) states: "The Spleen acts as the guard, facilitating the reception of grain. By observing the condition of the lips and tongue, one can discern auspicious or inauspicious outcomes." *Re Bing Pian* (Chapter on Heat Diseases) states: "In Lung Heat Disease, the tongue coating is yellow." This is because the Spleen governs muscles, Wei Qi (Defensive Qi) resides in the muscles, and the lips and tongue are the roots of the muscles; thus, observing them verifies the prognosis. Regarding physical strength, longevity, and all pathological conditions, one must particularly integrate findings from constitutions (Bingfu) and various disease categories to clarify the principles. Among these, pulse diagnosis is most detailed. Pulses reflect the circulation of Qi and Blood, involving ascending, descending, exiting, and entering movements. Therefore, they should be viewed in conjunction with Ying and Wei (Nutritive and Defensive Qi), meridians, Yin and Yang, and Zang-Fang organs. One must comprehend their subtle principles so that under the fingers, one can clearly distinguish between harmony and disease, deficiency and excess. Ancient masters said regarding pulse diagnosis: "What my mind understands, my mouth cannot fully express." These subtle principles require mental comprehension and cannot be transmitted merely through words. If one understands the pulses, the truth or falsity of syndromes becomes apparent; if one distinguishes the syndromes, the deficiency or excess of the pulses becomes clear. What is termed "True" is when pulses and syndromes coincide, making deficiency and excess obvious. What is termed "False" is when pulses and syndromes do not coincide, where Excess diseases mimic Deficiency, and Deficiency diseases mimic Excess. True conditions are easy to distinguish; False conditions are difficult. Failing to distinguish the false and treating erroneously can kill someone in the blink of an eye. Can one not revere this? Can one not be cautious!