古人制方之义，微妙精详，不可思议。盖其审察病情，辨别经络，参考药性，斟酌轻重
，其
于所治之病，不爽毫发。故不必有奇品异术，而沉痼艰险之疾，投之辄有神效，此汉以前之
方也。但生民之疾病，不可胜穷，若必每病制一方，是曷有尽期乎？故古人即有加减之法，
其病大端相同，而所现之症或不同，则不必更立一方，即于是方之内，因其现症之异，而为
之加减。如《伤寒论》中，治太阳病用桂枝汤，若见项背强者，则用桂枝加葛根汤；喘者，
则用桂枝加浓朴杏子汤；下后脉促胞满者，桂枝去白芍汤；更恶寒者，去白芍加附子汤，此
犹以药为加减者也。若桂枝麻黄各半汤，则以两方为加减矣。若发奔豚者用桂枝，为加桂枝
汤，则又以药之轻重为加减矣。然一二味加减，虽不易本方之名，而必明着其加减之药。若
桂枝汤倍用芍药而加饴糖，则又不名桂枝加饴糖汤，而为建中汤。其药虽同，而义已别，则
立名亦异。古法之严如此。后之医者，不识此义，而又欲托名用古，取古方中一二味，则即
以某方目之。如用柴胡，则即曰小柴胡汤，不知小柴胡之力，全在人参也。用猪苓、泽泻，
即曰五苓散，不知五苓之妙，专在桂枝也。去其要药，杂以他药，而仍以其方目之。用而不
效，不知自咎，或则归咎于病，或则曰咎于药，以为古方不可治今病，嗟呼！即使果识其病
而用古方，支离零乱，岂有效乎？遂相戒以为古方难用，不知全失古方精义，故与病毫无益
，而反有害也。然则，当何如？曰∶能识病情与古方合者，则全用之；有别症，则据古法加
减之；如不尽合，则根据古方之法，将古方所用之药，而去取损益之。必使无一药之不对症，
自然不倍于古人之法，而所投必有神效矣！
古人制定方剂的用意，精妙细致，难以用言语完全揣度。大概是因为他们仔细审查病情，辨别经络归属，参考药物性质，斟酌药量的轻重，对于所治疗的疾病，没有一丝一毫的偏差。所以不需要有奇特的药品或异样的医术，那些根深蒂固、险恶难治的疾病，一旦投药往往就能产生神奇的效果，这就是汉代以前的方剂特点。
但是百姓的疾病种类繁多，不可穷尽，如果每种病都专门制定一个方子，那什么时候才能结束呢？所以古人就有加减变化的方法：如果疾病的大致情况相同，但表现出的症状有所不同，就不必重新设立一个新的方子，而是在原方之内，根据现症的不同进行药物的增减。
例如《伤寒论》中，治疗太阳病（太阳经受邪所致的病证）使用桂枝汤（Cinnamon Twig Decoction），如果出现项背强急的症状，就使用桂枝加葛根汤；如果有气喘症状，就使用桂枝加厚朴杏子汤；如果经过攻下治疗后出现脉急促、腹部胀满的情况，就用桂枝去芍药汤；如果更加怕冷，就去掉芍药加上附子汤，这些都是以单味药为单位的加减。如果是桂枝麻黄各半汤，则是将两个方子合起来进行加减。如果是治疗奔豚病（一种气从少腹上冲心胸的病证）使用桂枝汤并增加桂枝用量，称为加桂枝汤，这又是以药物剂量的轻重来进行加减。
然而，仅仅增减一两味药，虽然不改变原方的名称，但必须明确标示出加减的药物。比如桂枝汤中加倍使用芍药并加入饴糖，就不叫桂枝加饴糖汤，而称为建中汤。虽然药物相同，但方义已经不同，所以命名也不同。古人的法度严谨到了这种程度。
后世的医生，不理解其中的深意，却又想借托古人之名来使用古方，只取古方中的一两味药，就称之为某某方。比如用了柴胡，就说这是小柴胡汤，却不知道小柴胡汤的功效全在于人参的使用；用了猪苓和泽泻，就说这是五苓散，却不知道五苓散的奥妙专在于桂枝的作用。去掉关键药物，混杂其他药物，却仍然沿用原方的名字。用药后不见效果，不知道反省自己，或者归咎于疾病，或者说归咎于药物，认为古方不能治疗现在的疾病。唉！即使确实认识了病情而使用古方，如果用法支离破碎、杂乱无章，怎么会有疗效呢？于是大家互相告诫说古方难以使用，殊不知这是完全失去了古方的精义，所以对疾病毫无益处，反而有害。
那么应该怎么做呢？回答说：如果能识别病情与古方相符的，就全方使用；如果有其他兼症，就依据古法进行加减；如果不完全符合，就根据古方的组方原则，对古方所用的药物进行取舍和损益。一定要做到没有一味药不对症，自然就不会违背古人的法则，而投药必然会产生神奇的效果了！
The intention behind the ancients' formulation of prescriptions was subtle, exquisite, and detailed, beyond full comprehension. This was because they carefully examined the condition of the disease, differentiated the meridians involved, referenced the properties of herbs, and deliberated on the dosage weights. Regarding the diseases they treated, there was not the slightest deviation. Therefore, there was no need for rare or extraordinary substances or techniques; yet, for chronic, intractable, and perilous ailments, administration of these formulas often yielded miraculous effects. These were the prescriptions of the pre-Han period.
However, the diseases afflicting the people are countless and inexhaustible. If a separate prescription were to be formulated for every single disease, when would it ever end? Thus, the ancients employed methods of modification (Jiajian). When the general nature of the disease was similar but the manifested symptoms differed, there was no need to establish an entirely new prescription. Instead, within the existing prescription, modifications were made based on the specific variations in symptoms.
For instance, in the *Shanghan Lun* (Treatise on Cold Damage), Guizhi Tang (Cinnamon Twig Decoction) is used to treat Taiyang disease (disease pattern caused by pathogenic invasion of the Taiyang meridian). If stiffness of the neck and back is present, Guizhi Jia Gegen Tang (Guizhi Decoction with Pueraria) is used; if wheezing occurs, Guizhi Jia Houpo Zizi Tang (Guizhi Decoction with Magnolia Bark and Apricot Seed) is used; if, after purgation, the pulse is rapid and the abdomen is full, Guizhi Qu Shaoyao Tang (Guizhi Decoction without Peony Root) is used; if aversion to cold worsens, Shaoyao is removed and Fuzi (Aconite Lateralis Radix Praeparata) is added. These are examples of modification by individual herbs. If one considers Guizhi Mahuang Ban Ge Tang (Half-Guizhi Half-Mahuang Decoction), this represents a modification combining two prescriptions. If Gui Zhi Gan Cao Long Gu Mu Li Tang (or specifically adding Gui Zhi to Guizhi Tang for Ben Tun syndrome) is used for Ben Tun (a condition characterized by gas rushing upward from the lower abdomen), this represents modification by adjusting the dosage weight of herbs.
Nevertheless, when modifying only one or two herbs, although the name of the original formula remains unchanged, the specific herbs added or removed must be clearly stated. For example, when Shaoyao (Peony Root) is doubled and Yi Tang (Maltose) is added to Guizhi Tang, it is not named "Guizhi Jia Yitang Tang" but rather Jianzhong Tang (Middle-Boosting Decoction). Although the ingredients may be similar, the therapeutic principle differs, and thus the name is different. The strictness of ancient methods was such that later physicians, failing to understand this principle yet wishing to claim authority by using ancient names, would take one or two herbs from an ancient prescription and label the entire mixture as that prescription. For example, using Chai Hu (Bupleuri Radix) leads them to call it Xiao Chai Hu Tang (Minor Bupleurum Decoction), unaware that the efficacy of Xiao Chai Hu Tang lies primarily in the use of Ren Shen (Ginseng Radix et Rhizoma). Using Zhu Ling (Polyporus) and Ze Xie (Alisma Rhizoma) leads them to call it Wu Ling San (Five-Ingredient Powder with Poria), unaware that the subtlety of Wu Ling San resides specifically in Gui Zhi (Cinnamomi Ramulus). By removing the key herbs and mixing in others, yet still applying the original name, they find the treatment ineffective. Not blaming themselves, they either blame the disease or the herbs, claiming that ancient prescriptions cannot treat modern diseases. Alas! Even if one correctly identifies the disease and uses an ancient prescription, if the application is disjointed and chaotic, how can there be efficacy? Consequently, people warn each other that ancient prescriptions are difficult to use, not realizing that this completely misses the essence of ancient formulations, thereby offering no benefit to the disease and causing harm instead.
So, what should be done? The answer is: If the patient's condition matches an ancient prescription, use the prescription in its entirety. If there are additional symptoms, modify it according to ancient principles. If it does not fully match, follow the principles of the ancient prescription to select, discard, increase, or decrease the herbs used. Ensure that no herb is mismatched to the symptoms. Naturally, this will not deviate from the methods of the ancients, and the administration of the medicine will surely yield miraculous results.