In Confucius's philosophical system, the meaning of "Du" (loyalty, sincerity, honesty, and singleness of purpose) is relatively simple, referring to loyalty, sincerity, honesty, and singleness of purpose.
In The Analects, the character "Du" (sincere devotion) appears seven times: The Master said, "When the Junzi (Exemplary Person) is devoted to his relatives, the people will be inspired toward Ren (Benevolence); when he does not forsake old friends, the people will not be fickle." (The Analects, Book VIII: Tai Bo)
The Master said, “Be steadfast in good faith and fond of learning; hold fast to the good Way until death. Do not enter a state in danger; do not stay in a state in chaos. When the Way prevails in the world, show yourself; when it does not, go into seclusion.” (The Analects, Book VIII: Tai Bo)
The Master said, “One who is praised for being sincere in speech—is this truly a Junzi (Exemplary Person), or merely one who puts on a solemn appearance?” (The Analects, Book 11, Chapter 20) Zi Zhang (a disciple of Confucius) asked about proper conduct. The Master said, “If your words are Zhong (Loyalty) and Xin (Trustworthiness), and your actions are sincere and respectful, then even in the lands of the Man and Mo barbarians, you will be accepted. But if your words lack loyalty and trustworthiness, and your actions lack sincerity and respect, then even in your own home village, how can you get along?” (The Analects, Book 15, Chapter 6)
Zi Zhang said: "If one holds onto De (Virtue) but does not broaden it, if one believes in the Dao (The Way) but is not steadfast, how can such a person be considered to exist? How can such a person be considered to have vanished?" (The Analects, Chapter 19, Zi Zhang)
Zi Xia (a disciple of Confucius) said: "Learn broadly and be steadfast in your will, inquire earnestly and reflect on what is near at hand—Ren (Benevolence) lies within this." (The Analects, Chapter 19: Zi Zhang)
The character "笃" (du) appearing in the above sentences all carries the meaning of "sincere," "genuine," and "earnest." Confucius advocated that a Junzi (Exemplary Person) should not only be "笃于亲" (devoted to relatives) but also be sincere in faith (笃信), firm in aspiration (笃志), and diligent in learning (笃学). Du (Sincerity) is a concept closely related to Xin (Trustworthiness) and Cheng (Authenticity). Confucius emphasized that in conducting oneself, one must value "Xin" (Trustworthiness). Xin is the foundation of establishing oneself in society, an important condition for governance, a standard for friendship, and a basic principle of moral cultivation. The concepts of Du and Xin are inseparable, and the qualities of "Zhongxin" (Loyalty and Trustworthiness), "Duhou" (Sincerity and Generosity), and "Zhengzhi" (Uprightness) inherent in them were often praised by Confucius.