Doctrine of the Mean

The Doctrine of the Mean (Zhōngyōng) is a book (often) attributed to be written by Zisi, the grandson of Confucius. What makes the Doctrine of the Mean different from for example the Analects is that it brings Heaven or Tiān into play.

Concepts

The "Mean" or Zhōngyōng talks about avoiding extreme emotions. You shouldn't be overly angry, nor apathetic. The doctrine emphasizes the importance of mmaintaining a balance of emotions. (Keep in mind: Balance does not equal mediocrity, instead you should be dynamic and apply the corresponding emotions depending on the situation. For example, courage should be the mean between fearfulness and recklessness)

Harmony with Heaven

Humans should align themselves with Heaven's Mandate or the cosmic order (Tiāndào). Heaven is the source of morality and the authority that gives legitimacy of rulers (Chinese emperors required to have Heaven's Mandate to legitimize rule). By aligning oneself, you can achieve inner peace and 'realize your true moral potential'.

The Noble Person

Junzi or the ideal person in the eyes of the Doctrine of the Mean should always act with wisdom, etiquette and righteousness. Junzi should also never lose their temper and maintain composure at all times. (again, this is not clear-cut and emotion should correspond to the situation)

Governance

A ruler should always practice the Mean so they rule benelovently to ensure a stable state. A ruler with Mean inspires goodness in others.

Read it online

The Doctrine of the Mean 中庸 - Translated by A. Charles Muller

The Doctrine of the Mean (and The Great Learning), translated by Robert Eno
(note for above: doctrine of the mean starts at page 22)