Mencius, which is the Latinized name of Meng-tze, was a disciple of the grandson of Confucius, and regarded as the second most important philosopher in the Confucian tradition. His work, known as the "Book of Mencius,"or "The Mengzi" is one of the four Shuh, or books, of Confucian wisdom.
The life of Mencius is very sketchy. Mencius was said to have lived to the advanced age of eighty-four years, which makes him a contemporary of the great Greek philosophers, Plato and Aristotle. His father died when he was very young and his education became the responsibility of his mother. Mencius was given a thorough instruction in the doctrine of Confucius. As an adult, Mencius became a minister of state to the feudal prince Hsüan. But the prince was not disposed to follow his counsel, so after a number of years, Mencius resigned his position and began traveling the states of China expounding the principles of Confucius. Finally, Mencius became advisor to Prince Hui, and helped the prince promote the welfare of his people. Following the death of Prince Hui, Mencius retired and spent his last years instructing his disciples, and preparing with them the book that bears his name.
The Book of Mencius is generally accepted as genuine, though the evidence of its authorship would hardly pass muster from contemporary historians. A Chinese history dating from 100 BCE., makes short account of Mencius’ life. He is also declared to be the author of the work in seven books that bear his name. There are extant portions of literary works composed as early as 186-178 BCE, containing quotations from the Book of Mencius. The book is regarded as one of the four essential texts of Confucian thought. Along with Analects of Confucius, the Mengzi joins the Daxue (The Greater Learning) and Zhongyong (The Center of Harmony).

The Mengzi

The main contents of The Mengzi, are the sayings of Mencius and his disciples. Mencius proposed the virtuous humanity theory: that humans are born with four moral characters -- humanity, justice, courtesy, and wisdom. Humans can lose these moral characters if they don’t maintain or develop them by practice. In this regard the Mengzi is similar in concept to Aristotle’s Ethics.
Mencius advocated what he called benevolent policy and kingcraft. Benevolent policy meant reducing penalties and taxes. Mencius also believed that that tyranny was the fundamental cause of suffering and misery.
The book is concerned with the regulation of human conduct from in respect to society and the state. Religion as a motive of right conduct seems to have concerned him much less than it did Confucius. He is interested in human conduct only in so far as it leads to the highest common good. Mencius was a believer in the Confucian principle that human nature tends to what is morally good, and only runs to evil by reason of the perverse influences of external environment. His treatise is one of the significant works to teach morality independently of religion.

The Mengzi argues that benevolence motivates us to love others, and that righteousness motivates us to fulfill our moral duties to others. Benevolence and righteouness enable us to follow the Way. If we act benevolently and righteously, then we can follow the Way. If we follow the Way of Heaven, then we can discover the goodness of which we are capable as moral beings. The Way is for each of us to be true to ourselves and to each other. If we are true to our sense of compassion for, and obligation to, others, then we can follow the Way, and we can each fulfill our proper destiny.
Mencius' ideas promote the roles of everyone within a society. Where he suggests that common people have roles and responsiblities to the broader society’s common good, Emperors and Princes have even more responsibility to act morally.  He thought that the emperor must attach great importance to the interest of ministers and common people; it is the responsibility of ministers to dissuade their emperor from making mistakes.

Mencius argues that a good ruler exhibits the Four Constant Virtues (benevolence, righteousness, propriety, and wisdom). Benevolent actions will bring honor to a ruler, while malevolent actions will bring disgrace to a ruler. If a ruler acts benevolently toward his subjects, then they will act benevolently toward him. If a ruler does not act benevolently toward his subjects, then they will not act benevolently toward him.

Perhaps unique for the time, Mencius also suggests that while Kings are divinely appointed the people have the right to overthrow a tyrant if he has not acted in the interests of the people. “Heaven sees according as the people see; Heaven hears according as the people hear,” he says. Therefore "the people are the most important element in a nation; the spirits of the land and the grain are the next; the sovereign is the lightest." Mencius states that the revolt against an unworthy sovereign is not rebellion but an act of righteousness.