A kind of ethical self-cultivation advanced by the Confucian school of thought, the term has two different meanings: First, du (独) is understood as at leisure and alone. When people are alone, without someone else’s supervision, they easily act in an undisciplined and immoral way. Shendu (慎独) requires being careful with one’s conduct when being alone, consciously following morality and the requirements of etiquette. Second, du is understood as an inner true state. People may in their words and actions manifest what is in accord with morality and the requirements of etiquette, but in their heart they do not accept or pursue any morality or etiquette. Shendu requires that one makes efforts in one’s heart, so that one’s inner world is in agreement with the words and actions required by morality and etiquette.
A man of virtue is cautious when he is not being watched by others and apprehensive when what he says is not being heard. There is nothing more visible than in what is secret, and nothing more obvious than in what is vague and minute. Therefore, a man of virtue is watchful when he is at leisure and alone. (The Book of Rites)
This means what one truly believes in his heart and mind will find expression in the open. That is why a man of virtue must be cautious when he is at leisure and alone. (The Book of Rites)