Emptiness and Twofold Mystery in Cheng Xuanying’s Commentary on the Meaning of the Daodejing: With a Discussion of the Equal Contemplation and the Forgetting of Things and Self

I. Introduction

This paper examines emptiness (kong 空) and Twofold Mystery (chongxuan 重玄) in Cheng Xuanying’s 成玄英 Commentary on the Meaning of the Daodejing (Daodejing Yishu 道德經義疏). Cheng was a Daoist priest of the Tang dynasty, and he left a significant mark on the history of Chinese thought. Some scholars divide the development of Daoist thought into Qin-Han Huang-Lao learning (Qin-Han Huang-Lao xue秦漢黃老學), Wei-Jin Learning of the Mysterious (Wei-Jin xuanxue 魏晉玄學), and Sui-Tang Twofold Mystery learning (Sui-Tang chongxuan xue 隋唐重玄學). Cheng is a representative figure of Sui-Tang Twofold Mystery learning. In particular, he exerted a profound influence on the integration of Daoism and Buddhism. In one of his major works, the Daodejing Yishu, Cheng interpreted the Laozi by drawing on Buddhism, and this had a major impact on the convergence of Daoism and Buddhism.¹ Emptiness and Twofold Mystery in the Daodejing Yishu are products of precisely this process. It hardly needs to be restated that emptiness is a core concept of Buddhism. Moreover, according to what has been confirmed so far, the first person to use the term “Twofold Mystery” was the Buddhist monk Zhidun 支遁 (314–66). (See Choi and Chung 2007, 16) When these points are taken into account, it is clear that emptiness and Twofold Mystery in the Daodejing Yishu are products of Daoist-Buddhist integration. Thus, an examination of emptiness and Twofold Mystery in the Daodejing Yishu has value as a way of clarifying the development of Daoist thought as well as the forms of Daoist-Buddhist integration that emerged in this process.

In examining emptiness and Twofold Mystery in the Daodejing Yishu, one point must be kept in mind: emptiness and Twofold Mystery should be treated in an integrated manner. This study demonstrates that emptiness and Twofold Mystery are closely connected in the Daodejing Yishu; emptiness constitutes the mode of being of the Dao, which transcends being and nonbeing (youwu 有無), while Twofold Mystery is the method for knowing the Dao, which exists in a mode that transcends being and nonbeing. In this sense, only by considering emptiness and Twofold Mystery together can we gain a fuller understanding of both concepts.

Previous studies on the Daodejing Yishu have generally not treated emptiness and Twofold Mystery together, but have instead focused primarily on Twofold Mystery. These studies share a dominant tendency: they focus on the logic of dialectical negation of being and nonbeing in Twofold Mystery. Previous scholarship is highly valuable in that it has analyzed the logical features of Twofold Mystery and clarified its significance within the history of thought.² However, because these studies have focused mainly on the logical features of Twofold Mystery, they have certain limitations. In addition to neglecting emptiness, they do not clearly identify Twofold Mystery as the method for knowing the Dao, whose mode of being transcends being and nonbeing. Furthermore, they do not sufficiently explain the state attained through knowing the Dao by means of Twofold Mystery. In the Daodejing Yishu, the state of knowing the Dao is presented as the equal contemplation of things and self (wuwo qiguan 物我齊觀) and the forgetting of both things and self (wuwo jianwang 物我兼忘). These ideas are significant in the history of thought because they point to a way of overcoming self-centeredness.

In light of these limitations, further investigation is needed. More specifically, Twofold Mystery should be examined in depth as the method for knowing the Dao and in relation to emptiness as the mode of being of the Dao.³ In the course of such an examination, it is also necessary to discuss the equal contemplation of things and self and the forgetting of both things and self as states of knowing the Dao.

This paper examines emptiness and Twofold Mystery in the Daodejing Yishu together. Section II examines emptiness as the mode of being of the Dao, which transcends being and nonbeing. Section III examines Twofold Mystery as the method for knowing the Dao, whose mode of being transcends being and nonbeing, and also discusses the equal contemplation of things and self and the forgetting of both things and self as states of knowing the Dao. In doing so, this paper aims to clarify the relatively underexplored concept of emptiness in the Daodejing Yishu, to deepen our understanding of Twofold Mystery, and ultimately shed light on the development of Daoist thought as well as the forms of Daoist-Buddhist integration that emerged within that development.

II. Emptiness as the Mode of Being of the Dao

Cheng Xuanying argues that all things are empty. Chapter 2 of the Daodejing Yishu reads as follows. Boldface in the original text and the translation of the quotations from the Daodejing Yishu below was added by the author to indicate passages from the original text of the Laozi.

It clarifies that all beings have no substantial reality (shi 實). Being (you 有) and nonbeing (wu 無) give rise to each other. In other words, the two names, being and nonbeing, are established in dependence on each other. If we infer and thoroughly examine principle (li 理) and nature (xing 性) on this basis, we can realize that they are empty. To know that being and nonbeing give rise to each other is sufficient to clarify that all beings have no substantial reality. The difficult (nan 難) and the easy (yi 易) complete each other. In an empty mind and wisdom-contemplation (huiguan 慧觀), there is neither ease nor difficulty. Yet in discriminating and attached feelings, there are both difficulty and ease. This is what it means to know that the difficult and the easy are established in dependence on each other. The long (chang 長) and the short (duan 短) reveal each other. If one reveals the long by means of the long, there is no long; if one compares the short with the short, there is no short. Therefore, one should know that the long and the short exist only by revealing each other. The high (gao 高) and the low (xia 下) contend with each other (qing 傾). “Qing” means to contend. In general, where there is the high, there is the low; if there is no low, there is also no high. Why is this so? If one compares a foot (chi 尺) with an inch (cun 寸), the foot is high; but if one compares a foot with a zhang (丈, ten chi), the foot is low. What was high a moment ago has now become low. Thus, the high and the low ultimately have no fixed form; rather, they contend with each other, and all are empty. Tone (yin 音) and sound (sheng 聲) harmonize with each other. In general, the gong tone (gongyin 宮音) and the shang tone (shangyin 商音), as well as stringed instruments and wind instruments, are completed through mutual harmony. If we infer and investigate nature (xing 性) and appearance (xiang 相) on this basis, we can realize that they all return to stillness (ji 寂). By extension, all existent things are likewise vacuous (xu 虛) and provisional (jia 假). Before (xian 先) and after (hou 後) follow each other. From the standpoint of the present, one looks at the past, and so there is the present; from the standpoint of the past, one looks at the present, and so it is called the past. The present is not present by itself, so how could there be the past? The past is not past by itself, so how could there be the present? If there is neither past nor present, what could be before and what could be after? The claim that there is before and after is ultimately without substantial reality, since the three times (sanshi 三時) follow one another.⁴

Cheng denies the substantial reality of all things—that is, their unchanging self-identity—on the basis of mutual dependence. He presents various pairs of relative entities, such as being and nonbeing, difficulty and ease (nanyi 難易), length and shortness (changduan 長短), high and low (gaoxia 高下), and before and after (xianhou 先後). These relative entities are mutually dependent. In other words, if one side of a relative pair does not exist, the other side cannot exist either. In Cheng’s view, these examples demonstrate that all beings are mutually dependent. Such mutually dependent beings cannot be said to possess substantial reality. A thing that cannot exist without its counterpart cannot be said to possess substantial reality. This is what Cheng means by emptiness.

The view that all things are empty on the basis of the mutual dependence of various relative entities cannot be found in the Laozi. The Laozi also points to the mutual dependence of all things, but it does not argue on that basis that all things are empty.

As is well known, this view has its intellectual origin in Buddhism. Buddhism had already explained the mutual dependence of all things in terms of dependent origination (pratītyasamutpāda; yuanqi 緣起), expressed in the formula: “Because this exists, that exists; because this arises, that arises; because this does not exist, that does not exist; because this ceases, that ceases.” As will be discussed later, since Nāgārjuna (Longshu 龍樹), dependent origination has been explained in terms of emptiness (śūnyatā; kong 空).

However, Cheng did not fully accept the Buddhist position. As will be discussed later, Cheng differs decisively from Buddhism in that he defines the essence of the Dao as nonbeing. Emptiness as understood in Buddhism is the negation of unchanging self-identity. Therefore, emptiness is not synonymous with vacuity (xu 虛) or nonbeing (wu 無). Even if the essence of a certain being were vacuity or nonbeing, if that essence remained unchanged, that being could not be called empty. This is because it would possess an unchanging self-identity in the form of vacuity or nonbeing. From the Buddhist perspective, nothing possesses an unchanging self-identity. Cheng, however, posits the essence of the Dao—that is, an unchanging self-identity. In this respect, his position differs decisively from that of Buddhism.

In Cheng’s view, among the various relative entities, being and nonbeing most clearly demonstrate that all things are empty. If one understands that being and nonbeing are mutually dependent, one can understand that all things lack substantial reality.

In particular, if one thoroughly examines principle (li 理) and nature (xing 性) in light of the mutual dependence of being and nonbeing, one can realize that they are empty. What, then, are principle and nature?

Cheng offers the following explanation of principle and nature. Chapter 62 of the Daodejing Yishu reads:

The Dao is the storehouse (ao 奧) of all things. The Dao is the subtle principle (li 理) that is vacuous (xu 虛) and yet penetrating (tong 通), and it is the correct nature (xing 性) of sentient beings (zhongsheng 衆生). “Ao 奧” means a deep and hidden place, and it also means a storehouse. The Dao generates all beings and embraces all sentient beings (hanling 含靈). Because it is so great that there is nothing it does not encompass, it is said to be the storehouse of all things.⁵

The principle and nature to which Cheng refers are none other than the Dao. According to Cheng, “the Dao is empty in its essence, yet it functions to generate all things. From this perspective, the Dao is the principle (li 理) underlying the generation of all things. Moreover, because the Dao’s generative function extends to every part of the phenomenal world, the Dao as the generative principle is immanent in each individual entity within the phenomenal world. From this perspective, the Dao is the nature (xing 性) of all things.”⁶ In short, the Dao is both the generative principle and the nature of all things.

To say that principle and nature are empty is no different from saying that the Dao is empty. For Cheng, the Dao is both principle and nature. Cheng thus argues, on the basis of the mutual dependence of being and nonbeing, that the Dao as principle and nature is empty.

The claim that the Dao is empty is consistent throughout Cheng’s thought. Chapter 1 of the Daodejing Yishu reads:

The ultimate Dao is profound and subtle, and its limits cannot be fathomed. It is neither nonbeing (wu 無) nor being (you 有), and it is neither annihilation (duan 斷) nor permanence (chang 常).⁷

According to Cheng, the Dao is neither being nor nonbeing, and neither permanence nor annihilation. As examined above, all things are empty because they cannot be determined by such categories as being and nonbeing, difficulty and ease, length and shortness, high and low, and before and after. If the Dao likewise cannot be determined by being and nonbeing or by permanence and annihilation, then the Dao too must be empty.

The view that the Dao is empty on the basis of the mutual dependence of various relative entities cannot be found in the Laozi. In particular, Cheng argues that the Dao is neither being nor nonbeing and neither annihilation nor permanence in explaining the emptiness of the Dao. This does not accord with the position of the Laozi. According to the Laozi, “the Dao is hollow (chong 沖),”⁸ which, in my view, corresponds to nonbeing. Moreover, according to the Laozi, the Dao is eternal—that is, permanent (chang 常).⁹

Cheng’s view that the Dao is empty on the basis of the mutual dependence of various relative entities has its intellectual origin in Buddhism. In particular, in arguing that the Dao is empty, he employs the logic of dialectical negation, a form of reasoning frequently used in Buddhism. Various Buddhist texts present a range of relative concepts and apply this logic to them, thereby denying the substantial reality of all things.¹⁰ A representative example is the Middle Treatise (Mūlamadhyamakakārikā; Zhonglun 中論). The “Chapter on Examining Conditions” (Guan yinyuan pin 觀因緣品) reads:

Neither arising (sheng 生) nor ceasing (mie 滅), neither permanence (chang 常) nor annihilation (duan 斷), neither identity (yi 一) nor difference (yi 異), neither coming (lai 來) nor going (chu 出). Whoever can teach these causes and conditions (hetu-pratyaya; yinyuan 因緣) skillfully removes various conceptual proliferations (prapañca; xilun 戲論). I bow my head in reverence to the Buddha, for this teaching is the foremost among all teachings.¹¹

Nāgārjuna explains causes and conditions through the Eight Negations (aṣṭa-niṣedha; babu 八不). From the Buddhist perspective, all beings arise through causes and conditions. Beings that arise through such causes and conditions are neither arising nor ceasing, neither permanent nor annihilated, neither identical nor different, and neither coming nor going. This dialectical negation of arising and ceasing, permanence and annihilation, identity and difference, and coming and going is precisely the Eight Negations.

The Middle Treatise presents various relative concepts and applies the logic of dialectical negation to them. In doing so, it demonstrates that all beings arise through causes and conditions and that, for this reason, all beings are empty. This point is further illustrated in the following passage. The “Chapter on Examining the Four Noble Truths” (Guan sidi pin 觀四諦品) of the Middle Treatise reads:

The existents (dharma; fa 法) that arise from various causes and conditions, I call emptiness (śūnyatā; wu 無). They are also called provisional names (prajñapti; jiaming 假名), and this is also the meaning of the middle way (madhyamā-pratipad; zhongdao 中道).¹²

According to the verse on the three truths (sandi ji 三諦偈), beings that arise through causes and conditions are identified with emptiness,¹³ provisional naming, and the middle way. Beings that arise through causes and conditions—that is, through various causes and conditions—cannot possess substantial reality or unchanging self-identity. In other words, they are empty. Such empty beings are conceptualized through language and exist on the linguistic level. In this sense, they are provisional names. In sum, beings that arise through causes and conditions are empty, yet at the same time they exist as provisional names. This is the meaning of the middle way.

According to the verse on the three truths, the middle way indicates that beings arising through causes and conditions are empty. As is well known, Nāgārjuna’s Eight Negations are called the Middle Way of the Eight Negations (babu zhongdao 八不中道) a paradigmatic form of the middle way, and the forms of dialectical negation found in various Buddhist texts are also closely related to the middle way. Thus, the dialectical negation found in Nāgārjuna’s Eight Negations and in many Buddhist texts can be understood as indicating that beings arising through causes and conditions are empty.

Cheng’s view that the Dao is empty resembles the Buddhist dialectical negation represented by Nāgārjuna’s Eight Negations. The relative concepts presented by Cheng and Nāgārjuna do not completely coincide. Nevertheless, both Cheng and Nāgārjuna present various relative concepts and apply the logic of dialectical negation to them. In addition, Buddhist dialectical negation, represented by Nāgārjuna’s Eight Negations, shows that beings arising through causes and conditions are empty, and Cheng likewise argues through dialectical reasoning that the Dao is empty. Taking these points into account, it is highly likely that Cheng’s view that the Dao is empty was influenced by Buddhist texts such as the Middle Treatise.

Here we must consider the precise meaning of the claim that the Dao is empty. If the statement that the Dao is empty means that the essence of the Dao as principle and nature is empty, then Cheng’s principle of the generation of all things would be no different from dependent origination, and the nature of all things would be no different from non-self (anātman; wuwo 無我). In that case, the Dao would amount to dependent origination and non-self. If one were to understand the Dao in terms of dependent origination and non-self, this would be Buddhism, not Daoism. It is true that Cheng partially accepted Buddhist theory, but as is well known, he did so in order to respond to Buddhist critiques of Daoism. Cheng’s ultimate aim was to criticize Confucianism, Buddhism, and other systems of thought that competed with Daoism and to establish Daoism as the ultimate truth. This is clearly demonstrated throughout the Daodejing Yishu. One clear example is its opening statement: “This Classic of the Dao and Its Virtue is the crown and cap (guanmian 冠冕) of the three teachings (sanjiao 三教) and the leader of the many scriptures.”¹⁴ Taking these points into account, it is unlikely that the statement that the Dao is empty means that the essence of the Dao as principle and nature is empty.¹⁵

In my view, the statement that the Dao is empty means, more precisely, that emptiness characterizes the Dao’s mode of being. Cheng frequently explains the Dao through various relative concepts, the most representative of which are being and nonbeing. These concepts do not explain the essence of the Dao but rather its mode of being. Taking this into account, it is highly likely that Cheng’s claim, expressed through various relative concepts such as being and nonbeing, that the Dao is empty refers to the emptiness of the its mode of being.

Cheng further explains the point in terms of being and nonbeing. Chapter 14 of the Daodejing Yishu reads:

It is the form of what has no form (wuzhuang zhi zhuang 無狀之狀) and the image of what has no thing (wuwu zhi xiang 無物之象). With regard to the form and appearance (zhuangmao 狀貌) and shape and image (xingxiang 形象) of the Dao, its subtle substance (miaoti 妙體) is not seen and not heard, and therefore it is called “having no form” and “having no thing.” Yet its phenomena (ji 迹) are generated and transformed, and therefore they are called “the form of” and “the image of.” This is called indistinct and obscure (huhuang 惚恍). “Indistinct and obscure” refers to a form that is not fixed. The subtle substance is not being, and its phenomena (yingji 應迹) are not nonbeing. Since it is neither being nor nonbeing, it is both nonbeing and being. Because being and nonbeing are not fixed, it is called “indistinct and obscure.”¹⁶

According to Cheng, the Dao transcends human senses such as hearing and sight. The Dao has both an aspect of substance and an aspect of phenomena. As substance, the Dao is neither heard nor seen, and therefore has no form. In this respect, the Dao is not being. By contrast, as phenomena, the Dao is a being that is generated and transformed,¹⁷ and therefore has form. In this respect, the Dao is not nonbeing. Because the Dao is neither being nor nonbeing, it is both nonbeing and being. This is because what is not being corresponds to nonbeing, while what is not nonbeing corresponds to being. The mode of being of the Dao is thus such that it cannot be determined as either being or nonbeing. Cheng describes this mode of being of the Dao as indistinct and obscure.

One point should be clarified here: Cheng uses the concepts of being and nonbeing in a dual sense. As noted above, Cheng argues that all things are empty through the concepts of being and nonbeing. In that case, being and nonbeing refer to relative entities in the phenomenal world. In addition to being and nonbeing, such relative entities in the phenomenal world include difficulty and ease, length and shortness, high and low, and before and after. All beings in the phenomenal world are empty because they are mutually dependent in this way. At the same time, Cheng argues through the concepts of being and nonbeing that the mode of being of the Dao is empty. In this case, being and nonbeing refer respectively to the Dao as phenomena and the Dao as substance. Since the mode of being of the Dao is thus not fixed as either being or nonbeing (youwu buding 有無不定), it is empty, without unchanging self-identity. In my view, Cheng maintains theoretical coherence in his dual use of the concepts of being and nonbeing.

In addition to being and nonbeing, Cheng also explains the mode of being of the Dao through the concepts of ancient and present (gujin 古今). Chapter 14 of the Daodejing Yishu states:

This clarifies that the Dao is neither being nor nonbeing, and neither ancient (gu 古) nor present (jin 今). When one meets it, one cannot see its head. This clarifies that the Dao is not ancient and has no beginning. When one follows it, one cannot see its back. This clarifies that the Dao is not present and has no end.¹⁸

According to Cheng, the Dao transcends time. The Dao is not something that existed in the past, nor is it something that exists in the present. Therefore, the beginning and end of the Dao’s existence cannot be known. The mode of being of the Dao is such that it transcends past and present in this way.

By presenting various relative concepts such as being and nonbeing, and ancient and present, and by applying to them a logic of dialectical negation, Cheng points out that the mode of being of the Dao has no unchanging self-identity. As already mentioned, Cheng is not pointing out that the essence of the Dao lacks self-identity. He clearly states that the Dao as substance is nonbeing.¹⁹ This nonbeing is the essence of the Dao, and this is the unchanging self-identity of the Dao itself. Cheng therefore acknowledges that the essence of the Dao has self-identity.

Cheng presents various relative concepts and applies a logic of dialectical negation to them in order to point out that the mode of being of the Dao is empty. This is, in essence, an adoption of the Buddhist logic of dialectical negation represented by Nāgārjuna’s Eight Negations.

Cheng’s adoption of the Buddhist logic of dialectical negation in order to point out that the mode of being of the Dao is empty was intended as a response to Buddhist critiques of Daoism. In the late Sui 隋 dynasty, slightly before Cheng’s period of activity, and in the early Tang dynasty, when Cheng was active, competition among the three teachings of Confucianism, Buddhism, and Daoism was intense. Each of the three teachings refined its own theories and criticized those of its rivals. A representative Buddhist critic of Daoism was Jizang 吉藏 (549–623). The Profound Meaning of the Three Treatises (Sanlun Xuanyi 三論玄義) states:

In the Dao of Laozi (Boyang 伯陽), the Dao refers to vacuity and nonbeing (xuwu 虛無). In the Dao of Śākyamuni, the Dao transcends the four alternatives (catuṣkoṭi; siju 四句). Since their shallowness and depth are already vastly different, how could their substance (ti 體) derive from the same source? . . . Although Laozi’s Dao leads the nine currents (jiuliu 九流) and encompasses the Seven Summaries (Qilüe 七略), it merely distinguishes being and nonbeing and does not clarify the severing of the four alternatives. If one claims that Laozi’s teaching also distinguishes double negation (shuangfei 雙非), this is roughly like mixing sand with gold, or like the claim of one who steals an ox.²⁰

According to Jizang, the Dao of Daoism is nothing more than vacuity and nonbeing, whereas the Dao of Buddhism transcends the four alternatives. The two are therefore fundamentally different. If one claims that Daoism also contains the transcendence of the four alternatives—that is, a logic of dialectical negation such as the transcendence of being, nonbeing, neither-being-nor-nonbeing, and both-being-and-nonbeing (yiyou yiwu 亦有亦無)—this is fraud and plagiarism. In short, Daoism is far inferior to Buddhism. As a Daoist priest, Cheng could not have ignored such Buddhist criticism of Daoism.

Cheng likely felt that responding to Buddhist criticism of Daoism solely on the basis of Daoist theory was limited. The core of the Buddhist criticism of Daoism was that the Dao of Daoism is merely nonbeing, whereas the Dao of Buddhism transcends both being and nonbeing. To respond to this criticism, it was necessary to show that the Dao of Daoism also transcends both being and nonbeing. However, Daoism did not possess a theoretical basis for claiming that the Dao transcends both being and nonbeing. Although Daoism has various relative concepts, including being and nonbeing, it does not possess a mode of thought that applies the logic of dialectical negation to such concepts. Cheng therefore chose to respond to Buddhist criticism of Daoism by adopting the Buddhist logic of dialectical negation.

Of course, Cheng could not have fully accepted Buddhist thought. To fully accept the claim that the Dao transcends both being and nonbeing would not accord with the religious and philosophical identity of Daoism. The claim that the essence of the Dao is vacuity and nonbeing is a core idea of the Laozi. If this were denied, it could no longer be called Daoism. It was important to respond to Buddhist criticism, but Cheng could not for that reason abandon Daoist identity.

Cheng therefore maintains the view that the essence of the Dao is nonbeing, while also presenting the idea that the mode of being of the Dao transcends being and nonbeing. This was possible because Cheng argued that the Dao has both an aspect of substance and an aspect of phenomena. As seen above, for Cheng, the Dao is ultimate nonbeing (zhiwu 至無). This is nonbeing as the ontological source of being, that is, the Dao as substance. The Dao as substance transcends human sensation. It is therefore not being. “The Dao as substance appears as phenomena by performing the function of generating all things. This is the Dao as phenomena” (You 2017, 250). The Dao as phenomena is the totality of all things generated by the Dao as substance, that is, the phenomenal world itself. It is therefore not nonbeing. In this way, because the Dao is neither being nor nonbeing, it is both nonbeing and being. In short, the Dao exists in a mode that transcends being and nonbeing. This claim clearly reveals the integration of Cheng’s view, which illuminates the Dao from the perspectives of substance and phenomena, with the Buddhist logic of dialectical negation.

That the mode of being of the Dao transcends being and nonbeing means that the mode of being of the Dao lacks unchanging self-identity. Cheng expresses this by borrowing the Buddhist term emptiness. This is emptiness as the mode of being of the Dao. Through the concept of emptiness as the mode of being of the Dao, Cheng argues that the Dao transcends being and nonbeing without violating the Laozi’s view that the essence of the Dao is vacuity and nonbeing. In other words, while maintaining Daoist identity, Cheng responded to Buddhist criticism of Daoism by adopting Buddhist thought. This is the significance of the concept of emptiness as the mode of being of the Dao within the history of thought.

III. Twofold Mystery as the Method for Knowing the Dao

The ultimate aim of Daoism is to know the Dao. How, then, can one know the Dao, which exists in an empty mode—that is, in a mode that transcends being and nonbeing? As a Daoist priest, Cheng must have felt a responsibility to answer this question. In fact, Cheng does present a method for knowing the Dao.

To begin with, Cheng points out that there are two perspectives from which one may look at oneself. Chapter 1 of the Daodejing Yishu states:

This reveals that the two kinds of seeing are not the same. Always without desire (wuyu 無欲), one contemplates its subtlety (miao 妙). “Subtlety” means what is refined and exquisite. “Contemplation” means to illuminate and examine oneself. This means that if a person can always be without desire, practice nonaction (wuwei 無爲), and be utterly vacuous (xu 虛) and utterly still (jing 靜), then, at a near level, one can illuminate and see the subtle Dao within oneself, and, at a distant level, one can illuminate and see the refinement of ultimate principle (li 理). Always with desire (youyu 有欲), one sees its return or outcome (jiao 徼). “Jiao” means the point of return or outcome. “Desire” refers to being defiled by emotions and turning things into objects (jing 境). This means that if a person cannot practice nonaction, be without desire, see subtlety, and preserve truth, but instead wrongly gives rise to craving and seeking, and is defiled and obstructed by unrestrained emotions, then one merely sees the being (you 有) of the objective world. One does not realize that the being of the objective world is empty, cannot examine the refinement of subtle principle, and sees only the course of death and life.²¹

First, there is the perspective of being without desire (wuyu 無欲) in looking at oneself. Here, being without desire means more than the absence of desire; it also means nonaction (wuwei 無爲). A state without desire may be described as a natural state from which artificiality has been excluded. In this respect, being without desire is essentially consistent with nonaction. Cheng expands the meaning of being without desire into nonaction on the basis of this point. Ultimately, the perspective of being without desire means looking at oneself through nonaction, that is, through a natural attitude free from artificiality. If one contemplates oneself through such a natural attitude free from artificiality, then, at a near level, one comes to recognize the Dao within oneself—that is, the Dao as nature (xing 性); and, at a distant level, grasps principle, that is, the Dao as the generative principle of all things. In my view, the claim that one can know the Dao as principle and nature means that one could apprehend the aspect of substance among the two aspects of the Dao as principle and nature, namely, the Dao as nonbeing. If the claim that one can know the Dao as principle and nature were taken literally, it would mean that one can know both the aspect of substance and the aspect of phenomena within the Dao as principle and nature—that is, both the Dao as nonbeing and the Dao as being. In that case, the context would not read naturally. As will be discussed below, Cheng argues that both the perspective of being without desire and the perspective of being with desire have limitations. More specifically, the perspective of being without desire is attached only to nonbeing, while the perspective of being with desire is attached only to being. If one is attached only to nonbeing, one is likely to know only the Dao as nonbeing; if one is attached only to being, one is likely to know only the Dao as being. Taking these points into account, the Dao that can be known through the perspective of being without desire is presumably the Dao as nonbeing. If both the Dao as nonbeing and the Dao as being could be known through the perspective of being without desire, Cheng’s claim that this perspective has limitations could not stand. In short, if one looks at oneself from the perspective of being without desire, one can know the Dao as nonbeing.

Second, there is the perspective of being with desire (youyu 有欲) in looking at oneself. Here, being with desire means more than the presence of desire; it means the inability to practice nonaction. A state with desire may be described as an unnatural state to which artificiality has been added. In this respect, being with desire is essentially consistent with action or intentional activity (youwei 有爲). On this basis, Cheng expands the meaning of being with desire into the inability to practice nonaction—that is, into action. Ultimately, the perspective of being with desire means looking at oneself through action, namely, through an unnatural attitude shaped by artificiality. If one contemplates oneself through such an unnatural attitude, one can know only being, that is, the Dao as the phenomenal world, and cannot know the Dao as principle. In my view, the claim that one cannot know the Dao as principle means that one cannot know the aspect of substance among the two aspects of the Dao as principle, namely, the Dao as nonbeing. If the claim that one cannot know the Dao as principle were taken literally, it would mean that one cannot know either the aspect of substance or the aspect of phenomena within the Dao as principle—that is, neither the Dao as nonbeing nor the Dao as being. In that case, Cheng’s claim would become contradictory, since he has already stated that if one looks at oneself from the perspective of being with desire, one can know the phenomenal aspect, that is, the Dao as being. In short, if one looks at oneself from the perspective of being with desire, one can know the Dao as being.

Next, Cheng argues that the Dao can be known by overcoming both the perspective of being with desire and the perspective of being without desire. Chapter 1 of the Daodejing Yishu states:

These two emerge together but have different names. “The two” refers to the two kinds of seeing: always being without desire and thereby seeing its subtlety, and always being with desire and thereby seeing its ultimate outcome. “Emerge together” means that they emerge together from the one Dao. “Have different names” refers to the distinction between ultimate outcome and subtlety. Originally, the object that is seen is only one, but there are differences in the wisdom of the seeing subject. Because the two kinds of seeing are not the same, ultimate outcome and subtlety have different names. Together they are called mystery (xuan 玄). “Mystery” means profundity; it also means nonattachment. The two minds, one with desire and one without desire, and the two kinds of seeing, one seeing the ultimate outcome and one seeing subtlety, are rooted in the one Dao. They emerge together from the one Dao yet have different names, and although they have different names, they come from the one Dao; therefore, they are called profound. Profound mystery, in terms of principle, returns to nonattachment: it is not attached to being (you 有) and not attached to nonbeing (wu 無). Because it is attached to neither of the two, it is called mystery. It is not only unattached to attachment, but also unattached to nonattachment: mystery and again mystery (xuanzhi youxuan 玄之又玄). Those with desire are attached to being, and scholars without desire are attached to nonbeing. Therefore, the single mystery (yixuan 一玄) was taught in order to make them abandon attachment to both. But because practitioners might become attached to this mystery, further mystery (youxuan 又玄) was taught in order to remove the later danger. This not only prevents attachment to attachment, but also prevents attachment to nonattachment. Since this is abandoning and again abandoning, it is called “mystery and again mystery.” It is the gate of all subtleties. “Subtlety” means what is so subtle that it cannot be fathomed. “Gate” means a Dharma gate (Dharma-paryāya; famen 法門). Earlier, by means of the mystery of the one middle (yizhong 一中), attachment to the two extremes was abandoned, and the illness of the two extremes had already disappeared. Then the medicine of the one middle was also abandoned, so that medicine and illness vanished together at once. This is precisely the supremely subtle and refined state of exhausting principle and fully realizing nature (qiongli jinxing 窮理盡性). How could it be merely the gate of the sages? It is also the Dharma gate of all subtleties.²²

According to Cheng, the difference between the perspective of being with desire and the perspective of being without desire arises from attachment to being and nonbeing. Both are perspectives from which one looks at oneself. However, the perspective of being with desire looks at oneself while attached to being, whereas the perspective of being without desire looks at oneself while attached to nonbeing. This is why the difference between the two perspectives arises.

In Cheng’s view, both being with desire and being without desire are perspectives that must be overcome. The perspective of being with desire is limited because it is attached to being, while the perspective of being without desire is limited because it is attached to nonbeing. The claim that the perspective of being with desire is attached to being is easy to understand. The claim that the perspective of being without desire is attached to nonbeing, however, is less straightforward. How can attachment to a particular object exist in a state without desire? This appears contradictory. Of course, there is no actual contradiction in Cheng’s claim. The statement that the perspective of being without desire is attached to nonbeing means that, in pursuing the absence of desire, one comes to be attached to the idea that there should be no desire. Cheng argues that one should not be attached to being, nor should one be attached to nonbeing. This is mystery—that is, the one middle (yizhong 一中).

Cheng does not stop with the presentation of mystery. This is because attachment to mystery still remains. Cheng argues that attachment to mystery must also be overcome. In other words, one must overcome not only attachment to being and nonbeing, but also the attachment to the idea that one must overcome attachment to being and nonbeing. This is further mystery—that is, Twofold Mystery.

According to Cheng, one can fully know the Dao through Twofold Mystery. More specifically, by overcoming attachment to being and nonbeing, and further overcoming even attachment to the idea that one must overcome attachment to being and nonbeing, one can fully know the Dao as principle and nature. This is the state of exhausting principle and fully realizing nature (qiongli jinxing 窮理盡性).

Cheng explains the state of knowing the Dao. Chapter 3 of the Daodejing Yishu states:

There is nothing that is not properly ordered (zhi 治). “Zhi” means to make correct. If a person engaged in cultivation can first abandon being with desire and then abandon being without desire, this means that one has abandoned both extremes, subtly realized the one Dao, equally contemplated things and self (wuwo qiguan 物我齊觀), and forgotten both the object and wisdom (jingzhi liangwang 境智兩忘). If one governs on this basis, there is nothing in one’s governing that is not properly ordered.²³

According to Cheng, a person who overcomes both being with desire and being without desire and thereby realizes the Dao comes to see all things and oneself equally, forgets both the object of cognition—that is, all things—and the wisdom of the cognitive subject—that is, oneself—and governs all things on this basis. In other words, a person who has realized the Dao overcomes discrimination and attachment concerning all things and oneself, and governs all things on the basis of this overcoming of discrimination and attachment. This is the state of realizing the Dao.

In this state, one is not attached to one’s own achievements. Chapter 2 of the Daodejing Yishu states:

The sage does not rely on or take credit for carrying out transformation (wei 爲). “Wei” means to carry out transformation. “Rely” means to trust and depend on oneself. In general, the sage empties his mind and benefits all things according to their capacities. He treats self and others equally (zita pingdeng 自他平等) and forgets both things and self (wuwo jianwang 物我兼忘). Therefore, even if he has great achievements, he never relies on himself, but forgets his own merit.²⁴

According to Cheng, the sage—that is, a person who has realized the Dao ²⁵—does not take pride in his own achievements, but forgets them. This is because, when the sage carries out transformation and benefits all things—that is, in governing ²⁶—he “treats self and others equally and forgets both things and self.” In this way, overcoming attachment to one’s own achievements is the completion of knowing the Dao.

For Cheng, “equally contemplating things and self and forgetting both object and wisdom” and “treating self and others equally and forgetting both things and self” do not remain merely on the level of self-cultivation. They are connected to the level of governing, or transformation through rule. In this respect, Cheng’s “equal contemplation of things and self, and forgetting both object and wisdom” and his “equality of self and others, and forgetting both things and self” differ decisively from Buddhism and are aligned with traditional Chinese thought before the transmission of Buddhism. In traditional Chinese thought, not only Confucianism but even Daoist thought. connect self-cultivation with governing.²⁷ By contrast, in Buddhism, although the bodhisattva path (bodhisattva-mārga; pusa dao 菩薩道), grounded in the thought of emptiness, does have the character of social practice, it is not, for that reason, an act of governing.

“Equal contemplation of things and self, and forgetting both object and wisdom” and “equality of self and others, and forgetting both things and self” are highly significant in the history of thought as forms of overcoming self-centeredness. The overcoming of self-centeredness occupies a central place in Chinese thought as a whole. Various intellectual traditions, including Confucianism and Mohism, advocated overcoming self-centeredness through ideas such as humaneness (ren 仁) and inclusive care (jian’ai 兼愛). Buddhism, which was transmitted from India and later came to occupy a mainstream position in the Chinese intellectual world, likewise advocated overcoming self-centeredness through ideas such as dependent origination (pratītyasamutpāda; yuanqi 緣起) and non-self (anātman; wuwo 無我). Taking these points into account, “equal contemplation of things and self, and forgetting both object and wisdom” and “equality of self and others, and forgetting both things and self” may be said to possess distinctive value in the history of Chinese thought as Daoist forms of overcoming self-centeredness.

The claim that the Dao can be known through Twofold Mystery is closely related to the claim that the mode of being of the Dao transcends being and nonbeing. If the mode of being of the Dao were either being or nonbeing, then the Dao could be known through either the perspective of being with desire, which is attached to being, or the perspective of being without desire, which is attached to nonbeing. However, the mode of being of the Dao is neither being nor nonbeing. Moreover, because it is neither being nor nonbeing, it is also both nonbeing and being. Therefore, the Dao cannot be known through the perspective of being with desire, which is attached to being, nor through the perspective of being without desire, which is attached to nonbeing. Nor can it be known through mystery, which is not attached to being and nonbeing. It can be known only through Twofold Mystery, which is not attached even to nonattachment to being and nonbeing.²⁸

What is known through Twofold Mystery is not emptiness as the mode of being of the Dao, but the Dao itself. In other words, it is the Dao as principle and nature. Knowing the Dao as principle and nature in this way is the state of exhausting principle and fully realizing nature. In this state of exhausting principle and fully realizing nature, one “equally contemplates things and self and forgets both object and wisdom,” and “treats self and others equally and forgets both things and self.” Merely understanding intellectually that the mode of being of the Dao is empty—that is, that the Dao exists in a mode that transcends being and nonbeing—does not mean that one can realize the Dao. To truly know the Dao, one must first overcome attachment to being and nonbeing, and then overcome the attachment to the idea of overcoming such attachment to being and nonbeing. This is Twofold Mystery. In this respect, Twofold Mystery has the character of a theory of self-cultivation.

As is well known, the term Twofold Mystery originates from the phrase “mystery and again mystery” (xuanzhi youxuan 玄之又玄) in Chapter 1 of the Laozi. However, the content of Twofold Mystery differs greatly from the original meaning of “mystery and again mystery.” The core of Twofold Mystery is a dialectical logic that negates both being and nonbeing, whereas “mystery and again mystery” was originally unrelated to such a logic of dialectical negation.

At this point, one may propose the hypothesis that the intellectual origin of Twofold Mystery lies in Nāgārjuna’s (Longshu 龍樹) Eight Negations (aṣṭa-niṣedha; babu 八不). The logic of dialectical negation, which forms the core of Twofold Mystery, is very similar to Nāgārjuna’s Eight Negations. In fact, Twofold Mystery and Nāgārjuna’s Eight Negations are closely related. Twofold Mystery is a method for knowing the Dao, which exists in a mode that transcends being and nonbeing, and Cheng’s account of the mode of being of the Dao that transcends being and nonbeing—that is, the concept of emptiness as the mode of being of the Dao—is based on Nāgārjuna’s Eight Negations. However, there is a decisive difference between Twofold Mystery and Nāgārjuna’s Eight Negations. Twofold Mystery repeats dialectical negation, whereas Nāgārjuna’s Eight Negations do not. Although the concepts of being and nonbeing do not appear in Nāgārjuna’s Eight Negations, if Nāgārjuna’s logic of dialectical negation is applied to the concepts of being and nonbeing, it yields only neither-being-nor-nonbeing—that is, the concept of mystery. It does not yield Twofold Mystery, which negates mystery once again. Taking these points into account, it is difficult to say that the intellectual origin of Twofold Mystery lies entirely in Nāgārjuna’s Eight Negations alone.

The intellectual origin of the logic that repeats dialectical negation in Twofold Mystery lies in Three Treatise learning (Sanlun xue 三論學). Jizang 吉藏, the great systematizer of Three Treatise learning, constructed a theory distinct from Nāgārjuna’s by employing a logic that repeats dialectical negation. This theory appears in his doctrine of the three kinds of two truths (sanzhong erdi shuo 三種二諦說) and the doctrine of the four levels of two truths (sizhong erdi shuo 四重二諦說).

Let us first explain the doctrine of the three kinds of two truths. This doctrine consists of three stages. The first stage is composed of being and nonbeing (kong 空).²⁹ At this stage, being is taken as the conventional truth (samvṛti-satya; sudi 俗諦), and nonbeing is taken as the ultimate truth (paramārtha-satya; zhendi 眞諦). The second stage is composed of duality (er 二) and nonduality (buer 不二). Duality refers to the two extremes. It includes various relative concepts, such as being and nonbeing, permanence (chang 常) and impermanence (wuchang 無常), ultimate truth (zhen 眞) and conventional truth (su 俗), and birth-and-death (samsara; shengsi 生死) and nirvana (nirvāṇa; niepan 涅槃). Nonduality refers to the middle way (madhyamā-pratipad; zhongdao 中道) of these two extremes. Jizang gives examples such as neither ultimate truth nor conventional truth and neither birth-and-death nor nirvana (fei shengsi fei niepan 非生死非涅槃). If one were to use being and nonbeing as an example, one could cite neither-being-nor-nonbeing (feiyou feiwu 非有非無). At the second stage, duality—that is, the two extremes—is taken as the conventional truth and nonduality—that is, the middle way of the two extremes—is taken as the ultimate truth. The third stage is composed of duality, nonduality, and neither-duality-nor-nonduality (fei er fei bu er 非二非不二). At the second stage, the two extremes such as being and nonbeing—that is, duality—are the conventional truth, and the middle way of the two extremes such as neither-being-nor-nonbeing—that is, nonduality—is the ultimate truth. From another perspective, however, duality is itself one extreme, and nonduality is also one extreme. Therefore, at the third stage, the duality that was the conventional truth at the second stage and the nonduality that was the ultimate truth at the second stage are both taken as conventional truth, while neither-duality-nor-nonduality, the middle way of duality and nonduality, is taken as the ultimate truth of the middle way (paramārtha-satya of the middle way; zhongdao diyi yidi 中道第一義諦).³⁰

Next, let us explain the doctrine of the four levels of two truths. This doctrine consists of four stages. The structure of the first through third stages is, in effect, identical to that of the doctrine of the three kinds of two truths. The fourth stage is composed of duality, non-duality, neither-duality-nor-nonduality, and the forgetting of language and the cessation of thought (yanwang lüjue 言忘慮絶). At the third stage, duality and nonduality are the conventional truth, while neither-duality-nor-nonduality, the middle way of duality and nonduality, is the ultimate truth. From another perspective, however, duality and nonduality are one extreme, and neither-duality-nor-nonduality is also one extreme. Therefore, at the fourth stage, the duality and nonduality that were the conventional truth at the third stage, together with neither-duality-nor-nonduality, which was the ultimate truth, are all taken as conventional truth, while that which transcends all such language and thought—that is, the forgetting of language and the cessation of thought—is taken as ultimate truth.³¹

Jizang’s doctrine of the two truths clearly displays a logic that repeats dialectical negation. This logic of repeated dialectical negation is distinctive to Three Treatise learning and is difficult to find not only in non-Buddhist systems of thought, but also in other schools of Buddhism. Yet Cheng’s Twofold Mystery also displays a logic that repeats dialectical negation. Taking this into account, it is highly likely that the intellectual origin of the repeated dialectical negation found in Cheng’s Twofold Mystery lies in Three Treatise learning as represented by Jizang.

Of course, there is a fundamental difference between Jizang’s doctrine of the two truths and Cheng’s Twofold Mystery. The doctrine of the two truths is a theory concerning the ultimate truth that transcends language—expressed by such terms as true truth (zhendi 眞諦), truth of the highest meaning (diyi yi di 第一義諦), and supreme truth (paramārtha-satya; shengyi di 勝義諦)—and the conventional linguistic truth of the world, expressed by such terms as conventional truth, worldly truth, and mundane truth (samvṛti-satya; shisu di 世俗諦). In other words, the doctrine of the two truths is a theory that clarifies what constitutes ultimate truth beyond language and what constitutes conventional linguistic truth in the world. Twofold Mystery, by contrast, is a method for knowing the Dao. Cheng also explains ultimate truth beyond language and conventional linguistic truth in the Daodejing Yishu,³² but that discussion is not directly related to Twofold Mystery.

Cheng established Twofold Mystery as a method for knowing the Dao by adopting not the Three Treatise view of truth itself, but rather its logic of repeated dialectical negation. Twofold Mystery as a method for knowing the Dao is ultimately a method for realizing the Dao. In the history of Daoist thought, the realization of the Dao is one of the central themes. For this reason, the history of Daoist thought contains a wide range of discussions concerning methods for realizing the Dao.³³ Taking these points into account, Twofold Mystery as a method for knowing the Dao may be said to possess distinctive value in the history of Daoist thought, as a method for realizing the Dao presented by Sui-Tang Twofold Mystery learning (Sui Tang chongxuan xue 隋唐重玄學), which was itself a product of Daoist-Buddhist integration.

IV. Conclusion

This paper has examined emptiness and Twofold Mystery in Cheng’s Daodejing Yishu. The main findings may be summarized as follows.

Cheng argues that the mode of being of the Dao is empty. In his view, if one understands that being and nonbeing (youwu 有無), difficulty and ease (nanyi 難易), length and shortness (changduan 長短), high and low (gaoxia 高下), and before and after (xianhou 先後) are mutually dependent, one can understand that all things are empty. In particular, if one thoroughly examines principle (li 理) and nature (xing 性) on the basis of the mutual dependence of being and nonbeing, one can understand that principle and nature are empty. This is the same as saying that the Dao is empty, because for Cheng, the Dao is both principle and nature. Here, the claim that the Dao is empty does not mean that the essence of the Dao is empty. If the essence of the Dao as principle and nature were empty, then the Dao as principle would be no different from dependent origination (pratītyasamutpāda; yuanqi 緣起), and the Dao as nature would be no different from non-self (anātman; wuwo 無我). To advocate the Dao as dependent origination and non-self would be Buddhism, not Daoism. The statement that the Dao is empty means that the mode of being of the Dao is empty. Cheng argues that the Dao exists in the mode of neither-being-nor-nonbeing and neither-ancient-nor-present (bugu bujin 不古不今). Since the mode of being of the Dao lacks unchanging self-identity in this way, it is empty. Cheng’s claim that the mode of being of the Dao is empty was intended as a response to Buddhist critiques of Daoism. Buddhism criticized Daoism by arguing that the Dao of Daoism is merely nonbeing, whereas the Dao of Buddhism transcends being and nonbeing, and that the Dao of Daoism is therefore inferior to the Dao of Buddhism. In response to this criticism, Cheng employed a logic of dialectical negation and argued that the Dao exists in the mode of neither-being-nor-nonbeing. In doing so, he countered that the Dao of Daoism also transcends being and nonbeing, without violating the Laozi’s view that the essence of the Dao is vacuity and nonbeing (xuwu 虛無). Cheng’s logic of dialectical negation was adopted from Nāgārjuna’s (Longshu 龍樹) Madhyamaka (Zhongguan xue 中觀學), represented by the Eight Negations (aṣṭa-niṣedha; babu 八不).

Cheng presents Twofold Mystery as the method for knowing the Dao. According to Cheng, there are two perspectives from which one may look at oneself: the perspective of being without desire (wuyu 無欲) and the perspective of being with desire (youyu 有欲). From the perspective of being without desire, one can know the Dao as nonbeing; from the perspective of being with desire, one can know the Dao as being. Yet both perspectives have limitations: the perspective of being with desire is attached to being, while the perspective of being without desire is attached to nonbeing. Cheng therefore argues that one should be attached to neither being nor nonbeing. This is mystery (xuan 玄), or the one middle (yizhong 一中). However, at the level of mystery, there still remains an attachment to the idea that one should not be attached to being and nonbeing. Cheng thus argues that even the attachment to the idea that one should not be attached to being and nonbeing must be overcome. This is further mystery (youxuan 又玄), or Twofold Mystery. Cheng holds that through Twofold Mystery one can know the Dao as principle and nature. This is the state of exhausting principle and fully realizing nature (qiongli jinxing 窮理盡性). A person who reaches this state “equally contemplates things and self, and forgets both object and wisdom” (wuwo qiguan, jingzhi liangwang 物我齊觀, 境智兩忘), and “treats self and others equally, and forgets both things and self” (zita pingdeng, wuwo jianwang 自他平等, 物我兼忘). Although this appears similar to the Buddhist position, it is aligned with traditional Daoist thought before the transmission of Buddhism in that it connects self-cultivation with governing. The claim that the Dao can be known through Twofold Mystery is closely related to the claim that the mode of being of the Dao transcends being and nonbeing. Because the mode of being of the Dao transcends being and nonbeing, the Dao cannot be known through the perspective of being with desire, which is attached to being, or through the perspective of being without desire, which is attached to nonbeing. Nor can it be known through mystery, which is not attached to being and nonbeing. It can be known only through Twofold Mystery, which is not attached even to nonattachment to being and nonbeing. Twofold Mystery displays a logic that repeats dialectical negation. This logic was adopted from Jizang’s 吉藏 doctrine of the two truths (satya-dvaya; erdi shuo 二諦說), which represents the great systematizer of Three Treatise learning (Sanlun xue 三論學).

Emptiness and Twofold Mystery in Cheng’s Daodejing Yishu are products of the development of the thought of the Laozi. In particular, they clearly reveal the forms of interaction between Daoism and Buddhism. Cheng actively adopted Buddhism both in responding to Buddhist critiques of Daoism and in constructing a Daoist theory of self-cultivation. In this process, he maintained the identity of Daoism while securing theoretical coherence between Buddhist and Daoist perspectives. Taking these points into account, Cheng’s work may be described not as a mere “mixture” of Daoism and Buddhism, but as a genuine “integration” of Daoism and Buddhism. In this respect, emptiness and Twofold Mystery in Cheng Xuanying’s Daodejing Yishu possess distinctive value in the history of thought.

Acknowledgments

This work was supported by the 2025 Research Fund of the University of Seoul.

References