天藏德。而以日为光明。人心藏神。而以阳气为固密。阴阳之道。必有所先养生之本。亦必有所谨。此内经原病之
所起。必眷眷于阴阳之论也。而又曰阴阳之要。阳密乃固。夫人身之阴阳皆欲固也。而必曰阳密乃固。其道维何。盖阳
者皆气而近浮。浮则在上。故曰阳因而上。然阳有高明之体。高明在上。此为真阳。而不可谓之浮阳必散而在外。散则
周遍。故曰阳以卫外。然阳有纯一之道。纯一而健。此为纯阳。而不可谓之外。非浮与外。则阳有元亨之隆也。不外不
浮。则阳有利贞之用也。是故阳之积。运之以生神明。而充奉之。是以精则养神。阳之运。倡之以为物先。而煦妪之。
是以柔则养筋。斯则阳之所事也。而必以密为固者。阳非不能固也。其失在不密也。致不密者有三。起居如惊。而神气
乃浮也。措情躁扰。与物骀宕。阳乃飞越而不归。则内之恬愉失。而元府不闭。风寒暑湿遂乘之以起。故外无御侮。内
必受兵。此不密之故一。烦劳则张而精绝也。夫阴为精。藏精而起。亟以赴阳。人若不知节息。每强力用之。且烦且劳。
烦则不静。劳则不息。而阳乃张矣。张如弓之久满。而不知弛。则弓力竭而筋干为伤。故精绝。驯致其道。必至目盲耳
背。溃若坏都。此不密之故二。大怒则形气绝。而血菀于上也。大怒则伤阳。阳既郁逆。则无所行。而菀于上。故有吐
血数升而殒者。有疽发于背者。皆薄厥之至也。此不密之故三。三者一起于外。而外得侮之。一困于内而内竭。为一乘
于猝而暴厥焉。则何能精则养神。而柔则养筋哉。然则如何而密。曰阳气者若天与日。天藏德而以日光明。则当
清HT 以
宁心。固精以养神。节劳以养筋。而阳倡阴和矣。阳健运则阴奠定。夫是之谓能固。
天（自然界）蕴藏德性，而依靠太阳来显现光明。人心蕴藏神志，而依靠阳气来保持固密。阴阳的道理，必然有所侧重；养生的根本，也必然有所谨慎。这正是《内经》探讨疾病起源的出发点，必定深切关注于阴阳的论述。
又说：“阴阳的要旨，在于阳气致密，阴气才能固守。”人身上的阴阳都想要固守，但特别强调“阳密乃固”，其中的道理是什么呢？大概阳气都是气体，性质轻浮而上行；轻浮则位于上部，所以说“阳因而上”。然而阳气具有高明的本体，高明位于上部，这称为真阳，而不能称之为浮散之阳。阳气必须散布在体表外部，散布则周遍全身，所以说“阳以卫外”。然而阳气有纯一的特性，纯一而刚健，这称为纯阳，而不能称之为外散之阳。如果不是浮散和外散，那么阳气就有元亨利通的盛大状态。如果不外散、不浮越，那么阳气就有利贞（稳固正固）的作用。因此，阳气的积聚，通过运化产生神明，并充盛奉养它，所以“精则养神”；阳气的运行，倡导作为万物的先导，并温煦抚育它，所以“柔则养筋”。这就是阳气所承担的功能。
之所以必须以“密”来实现“固”，是因为阳气并非不能固守，其失误在于不“密”。导致不“密”的原因有三点：一是起居作息如受惊扰，导致神气浮越；二是情志烦躁扰乱，与外界事物放纵懈怠，导致阳气飞越而不归位，从而内部恬静愉悦的状态丧失，玄府（汗孔及气机出入之门）不闭合，风寒暑湿便趁机侵袭而起。这样，外部没有抵御外邪的能力，内部必然遭受病邪侵害。这是不密的第一个原因。
二是烦劳过度导致阳气亢张而精气枯竭。阴就是精，收藏精气并频繁地供给阳气使用。人如果不懂得节制休息，常常勉强用力使用身体，既烦躁又劳累。烦躁则不得宁静，劳累则不得休止，于是阳气亢张。亢张就像弓拉满很久不知道放松，那么弓力耗尽，筋脉干枯受伤。所以精气枯竭，顺着这个趋势发展下去，必然导致目盲耳聋，身体溃败如同堤坝崩塌。这是不密的第二个原因。
三是大怒导致形体之气断绝，血液郁积于上部。大怒会损伤阳气，阳气既然郁结逆乱，就无法正常运行，而郁积于上部。所以有的人吐血数升而死，有的人背部发生痈疽，这都是薄厥（气血上逆导致的昏厥）严重的表现。这是不密的第三个原因。
这三种情况，一种起于外部，导致外部被邪气侵犯；一种困于内部，导致内部精气竭绝；一种乘于突然之间，导致暴发性昏厥。在这种情况下，怎么能做到“精则养神”和“柔则养筋”呢？
那么，怎样才能做到“密”呢？回答说：人体的阳气就像天空和太阳一样。天空蕴藏德性而依靠太阳显现光明，所以应当清心宁神，固护精气以养神，节制劳累以养筋，从而使阳气倡导、阴气和顺。阳气健康运转，阴气就能安定稳固。这就叫做能够固守。
Heaven stores virtue, and relies on the sun to manifest brightness. The human heart stores Shen (Spirit), and relies on Yang Qi (Yang Energy) to maintain solidity and density. The principle of Yin and Yang necessarily has a priority; the root of health preservation also necessitates caution. This is the origin of disease exploration in the *Neijing* (*Inner Canon*), which must earnestly focus on the discourse of Yin and Yang.
It further states: "The essence of Yin and Yang lies in Yang being dense, thereby ensuring Yin is solid." Both Yin and Yang in the human body desire to be solid, yet it specifically emphasizes "Yang density ensures Yin solidity." What is the rationale? Generally, Yang consists of Qi that is light and floating, tending upward; thus, it is said that "Yang ascends due to its nature." However, Yang possesses a sublime and bright essence located superiorly; this is called True Yang, not Floating Yang. Yang must disperse outward to the exterior; upon dispersing, it pervades the whole body; thus, it is said that "Yang protects the exterior." Yet, Yang follows a path of purity and unity; pure and vigorous, this is called Pure Yang, not External-Dispersed Yang. If not floating or externally dispersed, Yang possesses the grandeur of Yuan Heng (Supreme Prosperity). If not external-dispersed or floating, Yang exhibits the utility of Li Zhen (Beneficial Perseverance). Therefore, the accumulation of Yang, through operation, generates Shen Ming (Divine Illumination/Spiritual Activity) and nourishes it; hence, "when Jing (Essence) is refined, it nourishes Shen." The operation of Yang leads as the precursor of all things and warms them; hence, "when soft, it nourishes the sinews." These are the functions of Yang.
Why must "density" achieve "solidity"? It is not that Yang cannot be solid, but rather that the failure lies in lacking density. There are three causes for lacking density: First, daily life disturbances cause Shen Qi (Spiritual Qi) to float. Second, emotional agitation and lax interaction with objects cause Yang to fly away and fail to return, leading to the loss of internal tranquility and joy, and the failure of Yuan Fu (Mysterious Gates/Pores) to close. Consequently, Wind, Cold, Summer-heat, and Dampness take advantage to arise. Thus, there is no defense against external aggression, and internally, one must suffer attack. This is the first cause of lack of density.
Third, excessive mental and physical fatigue causes Yang to expand while Jing (Essence) is exhausted. Yin is Jing; Jing is stored and frequently supplied to Yang. If one does not know how to rest and constantly exerts oneself despite fatigue and agitation, Yang becomes hyperactive. Hyperactivity is like a bowstring kept drawn too long without release; eventually, the bow's strength is exhausted, and the sinews dry up and are injured. Thus, Jing is exhausted. Following this trajectory inevitably leads to blindness and deafness, and bodily collapse like a breached dam. This is the second cause of lack of density.
Fourth, extreme anger causes the form and Qi to sever, and blood stagnates in the upper part. Extreme anger injures Yang; once Yang is stagnant and rebellious, it cannot circulate properly and stagnates in the upper region. Hence, some may vomit large amounts of blood and die, or develop carbuncles on the back; these are severe cases of Bo Jue (Blood Stagnation Syncope). This is the third cause of lack of density.
These three factors: one originates externally, allowing external pathogens to invade; one exhausts internally; one strikes suddenly causing violent syncope. Under such conditions, how can one achieve "Jing nourishes Shen" and "Softness nourishes sinews"?
So, how does one achieve density? The answer is: Human Yang Qi is like Heaven and the Sun. Just as Heaven stores virtue and uses the Sun to provide light, one should clear the mind to calm the Heart, consolidate Jing to nourish Shen, and moderate labor to nourish sinews, so that Yang advocates and Yin harmonizes. When Yang operates vigorously, Yin settles firmly. This is called achieving solidity.