There has been considerable discussion over the centuries about the exact meaning of the title of the fourth of the Confucian Four Books, the Zhong Yong. The phrase derives from the Analects (VI.29), which is quoted in the book. Zhong means something like 'undeviating, neither to one side nor the other, neither too much nor too little', and is the foundation for the most common English title (due to Burton Watson), The Doctrine of the Mean; Gardner suggests 'maintaining perfect balance'. Early commentators, at least according to Legge, tend to understand yong to mean 'employment' or 'use', but later commentators tend to interpret it as meaning 'ordinary, constant'. Latin translators have tended to translate the whole title as De medio sempiterno; Legge suggests it would be less misleadingly called The States of Equilibrium and Harmony, taking zhong and yong as coordinate terms.
The Doctrine of the Mean, like The Great Learning, was originally a chapter in the Book of Rites, although as a later addition it seems to have had a long history of commentary treating it as a standalone work. When read as one of the Four Books, it is usually taken with Zhu Xi's comments and chapter divisions. Zhu Xi argued that the book had a Confucian provenance, being written by Zisi, who was Confucius's grandson; this gives the Four Books a nice symmetry, since it lets one say that we have here a series of four teachers; Confucius (Analects) hands down teaching to Zengzi (Great Learning) who hands down teaching to Zisi (Doctrine of the Mean) who hands down teaching to Mengzi(Mencius). Zhu Xi's arguments for attributing the work to Zisi have never been universally accepted, although they did become the standard view.
The Doctrine of the Mean can be read online in James Legge's translation at the Chinese Text Project. I am beholden also to the excerpts and commentary of Daniel K. Gardner's The Four Books and to James Legge's notes on the work.
The Neo-Confucian reading of the work, deriving from the Cheng brothers and becoming popular due to Zhu Xi, was to see it as having an egress-regress structure: it begins with a single principle, takes in the universe, and then returns it all to the principle again. The first chapter begins with essential ideas. Nature is what Heaven establishes, following nature is the Way, cultivating the Way is education. Following the Way requires not deviating, which requires close vigilance over one's own life. Human beings move from a tranquility prior to the passions to a unity of the passions, each with its proper proportion; these are Legge's 'equilibrium' and 'harmony'.
According to Zhu Xi, the next ten chapters illustrate with quotation the principles outlined in the first chapter. (Chapter II, incidentally, is odd in that it refers to Confucius in a nonstandard way, for no known reason.) The noble take into account circumstances in order to be constant (2), and this is the highest achievement, one few are able to reach in these days (3). Why are few able to reach it? Because those with talent think it beneath them, too simple to study, and those without talent think it above them, too hard to practice (4). The example of Shun shows the good that can come of giving it its proper attention (6). He did it by taking advice, drawing out the good from it, and applying it without deviation. This is precisely what the mean is, and precisely what made Shun the hero he was. No matter how much people might insist that they know how to live their lives, the results show otherwise: they may choose to hold to the Way, but they swiftly begin to deviate (7). They are not like Yan Hui, who made the choice and seized it firmly (8). People can do apparently difficult things with hard work and patience, but seem to have difficulty with this very fundamental thing (9).
The strength that sets others in order comes in more than one type (10). It can involve patience and reasonableness in conduct, and the power to endure hardship, even death. These are the traits of the noble in their pursuit of good and their resistance of bad. When we look for who is able to do these things and keep to the mean, we find that people often fall short or overshoot; in order to find a model, we need to look to the sage (11).
to be continued