“性与天道”问题在春秋战国之际备受各派思想家关注，《中庸》杰出的理论贡献就在于，它一方面以生生化育言天道，深刻影响了此后儒家天论的大方向，另一方面，它以性为天道之个体化与落实，揭示了天与万物之性的内在一致性，并将这种一致性归结为“诚”。“诚”因而成为《中庸》全篇的枢纽和理解其思想的关键。“诚”字在《中庸》中包含两重含义：真实无妄义和真实拥有义。这两层含义都是指事物及人对自己本性的拥有和实现，也即人与物在其实存中与自身本质相符合。《中庸》特别提出，人道不同于天道而又特出于万物之上的特质在于“诚之”，也即人可以通过“修道”即“教”的努力使自己突破形质的局限而实现天人合一，参赞天地之化育。因此《中庸》的“诚”论内在地包含着由天道下贯为人性和由人性上达天道的双向互动历程。从天道的意义上讲，《中庸》在其首章指出，中和是实现天地位，万物育的根本，而在后面对天地覆载化育万物的讨论中，则将此归本于“至诚无息”。《中庸》还认为，诚是物存在和延续的内在根据，贯彻物之终始。就人道而言，人道之“诚”上合天德则有“自诚明”之尽性和“自明诚”之致曲两个层次。天道之诚落实于性，故人可以即性显诚，从而昭明其德，但从更广泛的层面说，人必须通过“择善固执”的修道为学过程来实现人向天道的回归和超越。这一过程又包括两个层次：一方面，通过诚中形外的慎独工夫，以臻其生命之内外一体之效；另一方面，通过成己成物的忠恕之道，以开启合外内的明体尽性之道。有了天人合一的天道本体保障，再经过内圣外王的人道诚之工夫，人最终便能够实现至诚境界，与天地参。《中庸》的“诚”论体现了先秦儒家思孟学派究天人、合内外、即情显性的理论特色，对后世儒学尤其是宋明理学产生了重大启迪作用。深入研究《中庸》之“诚”论，对把握儒家的思想脉络至为重要，对今天中国哲学的研究和当代中国文化建设也有重要意义。

The topic of the relationship between humanity and Heaven receives great attention from various schools of thoughts in Spring-Autumn and Warring States Period. In this background, the excellent contribution of the Doctrine of Mean lies in that on one hand, it interprets Heaven in term of its participation in the promotion of development of things, on the other hand, it argues humanity as the manifestation of Heaven, indicating the internal consistence between Heaven and humanity, and attributing the cause of this consistence to Cheng. Therefore Cheng becomes the core concept of the Doctrine of Mean and crucial for us to understand the text.In the book Doctrine of Mean, Cheng has two kinds of meanings, one reality, the other real possession. Both the two meanings refer to the things and humans’ possession and realization of their own nature, which means that humans and things act in accordance with their own nature in their reality. The Doctrine of Mean stresses that the feature of humanity which is distinct from Heaven and superior to things consists in that human can break the limit of body by cultivation to reach the unity with heaven and participate in the development of things. Thus the theory of Cheng in the Doctrine of Mean inherently contains the interactive courses of becoming human from heaven and reaching heaven from humanity. From the sense of Heaven, the Doctrine of Mean points out harmony as the foundation for things to get their due place and for them to develop, while in later discussion of the function of Heaven in the growth and development of things, it attributes this role to supreme Cheng. The Doctrine of Mean also holds that Cheng serves as the internal foundation throughout the existence of things. With regard to humanity, Cheng falls into two levels. One is to fulfill humanity in adherence to Heaven; the other is to make investigation into specifics. Since Heaven exhibits itself in humanity, humans can achieve Cheng in their humanity, but in a broader sense, humans have to realize their return to Heaven by cultivation which process can be divided into two levels. On the one hand, we can fulfill the unity of inner thoughts and outer behavior by sticking to conscience despite staying alone. On the other hand, we can achieve the completion of humanity by doing our utmost and putting ourselves in other’s place. With unity of Heaven and humanity, humans can finally reach the realm of supreme Cheng through the cultivation of becoming sage inside and king outside. The theory of Cheng in the Doctrine of Mean demonstrates the features of the Pre-Qin Confucian Zisi-Mencius School which lays emphasis on the relationship between Heaven and humanity, providing inspiration for the development of the Song-Ming neo-Confucianism. In this way, to make deep research into Cheng in Doctrine of Mean not only carries crucial importance for our mastery of the path of Confucianism, but is very meaningful for today's research in Chinese philosophy and contemporary cultural construction in China.