Sincerity is among the core concepts of the Confucian school of thought. Basically, it means truthfulness without deceit. Confucians believed that sincerity is the essence of the “way of heaven” or “principles of heaven,” a basis on which everything else is built. At the same time, sincerity is also the root and foundation of morality. All moral deeds must be conducted on the basis of sincerity from the bottom of the heart. Otherwise, they are nothing but pretensions.The Doctrine of the Meanmaintains, “Nothing can be achieved without sincerity.” Sages are sincere by nature. Therefore, their words and deeds are naturally consistent with the “way of heaven” and the “principles of heaven.”Junzi(a man of virtue) upholds sincerity as his goal for moral attainment and an approach to achieving the “way of heaven” and the “principles of heaven.”

Citations：

Being as it is is the way of nature; being true to human nature is the way to achieve self-refinement. (The Book of Rites)

Sincerity means utter truthfulness without any pretensions or deceit. It is the natural state of the principles of heaven. (Zhu Xi:Annotations on The Doctrine of the Mean)