As a Confucian philosopher, Mengzi held in high esteem virtues that he called the "four principles" (siduan 四端), namely kindheartedness (ren 仁), appropriate behaviour (yi 义), etiquette (li 礼), and wisdom (zhi 智). Yet while Confucius was a kind of idealist, Meng Ke can be seen as more practical, and as the more aggressive of the two. Meng Ke did not shy away from conflicts with representants of other schools, and he even dared to criticize kings. He was, compared to Confucius, more explicit in the explanation of the term of "kindheartedness" or "humanity" (ren).

Benevolent government (renzheng 仁政). Kindheartedness was, in his eyes, an instrument of a ruler, who despised himself and saw himself as a servant to his people. He acted on behalf of the people, which was protected by Heaven. The personal behaviour of personal kindheartedness, which Confucius had spoken of, was thus by Mengzi extended into the field of government. A benevolent government was the true "way of the king" (wangdao 王道), expressed in low taxes (bo shui lian 薄税敛), austerity in lifestyle, and sparingly used punishment (sheng xing fa 省刑罚). The ruler had to care for sufficient grain so that the people would serve their parents and nourish their family and to live a life of happiness. A ruler was a such because he was able to "protect the people" (bao min 保民) and because the people was the most important item on his agenda (min wei gui 民为贵), the grain altars (sheji 社稷) being the second part, and lordship only of minor importance (jun wei qing 君为轻). Rulers not exerting the Confucian way of the king and ruling in the way of a hegemon (ba dao 霸道) had to be admonished by their ministers, and it was even the duty of the minister to remonstrance against cruelty in government, in the worst case, even to kill the tyrant. The ruler himself had to be obedient to his virtual father, Heaven. If he did not conduct government of benevolence, Heaven would express his anger by sending floods and natural disasters, and also directly through the people, which would leave the country of the tyrant, or rebel against him. Kindhearted government had to begin with a just distribution of land to all people so that they were able to live from their fields and the animal on their farm. Only if the people was guaranteed these basic needs, there would be place for the virtues of filial piety, loyalty, and trustworthiness.

The goodness of human nature. Another important philosophical issue of Mengzi is the goodness of human nature (ren xing shan 人性善). All humans have by nature a compassion for others and will not see them suffering (bu ren ren zhi xin 不忍人之心). If a child falls into a well, everbody would instinctively hurry to save it. The ruler, accordingly, would not dare see his people suffering hunger and cold, while "his own stables are full of well-fed horses". The inherence of goodness makes it possible that everybody is able to become a perfect saint, because he has an innate knowledge about what is good (liang zhi良知) and therefore the potential to do good things (liang neng 良能). Mengzi compares this innate knowledge with water that will, following the natural laws, flow downwards. Similarly, man will become good without doubt, if only he practices this goodness in an appropriate way. The potential to become a sage ruler like Yao 尧 or Shun 舜, Mengzi says, is given to everybody, even without adhering to a teacher, without having to study it, and without contemplating about it. Everybody possesses the natural access to moral values, which are bestowed upon him by Heaven, like kindheartedness (ren), righteousness (yi "an instinct of what is right or appropriate"), a need for ritual and etiquette (li), and knowledge (zhi) of what is good and what not. These four innate virtues (si de 四德) have to be enshrined in the heart (cun xin 存心) and nourished in the character (yang xin 养性), especially by persons of higher standing, i. e. the rulers of a state. The virtues yiand zhi had only had minor importance for Confucius, and were elevated to a higher position by Mengzi, as one of the four virtues. The have a concrete expression in human behaviour, the "four expressions of feeling" (si xin 四心) or "four branches of the mind" (si duan), namely compassion (ceyin 恻隐, as expression of kindheartedness), feeling ashamed (xiu'e 羞恶, as expression of rightousness), giving precedence (cirang 辞让, as expression of propriety), and an instinct for right and wrong (shifei 是非, as an expression of wisdom). Because everybody disposes of these feelings, they came be compared to the four limbs of the body (si ti 四体). Of these four, kindness and righteousness are the more important, the first being the feeling between two persons, and the latter the reverence towards a senior person. Kindness is a matter of the inner feelings, while righteousness is an outer expression towards the other. If the ruler acts kindheartedly and righteous, he will serve as a shining example to the whole people. The superior of the ruler is Heaven. In many instances, Mengzi stressed, it is not possible to enjoy a personal profit and simultaneously behaving in a moral way. The righteous man had in such cases rather to abandon his profit, or even his life, in order to behave morally correct.

Mengzi suggested several methods to achieve this Heavenly perfectness. Man had to "exhaust his heart" (jinxin) to find the Heaven-bestowed perfectly good character in it. He had to preserve this heart and to nourish this character, in order to serve Heaven. A kind of self-cultivation will help fostering this goodness throughout one's life, so that youth and old age perfectly meet each other. Man had furthermore to restrict his desires. Austerity will make him all the more rich, and not deprive him in any way. He had furthermore to care for that his heart would not go astray and leave the path of propriety. Sincerity (cheng 诚) is, as Mengzi says, an ideal instrument to travel on the human, and therefore, also the Heavenly way (tian zhi dao 天之道). Honesty or sincerity is not one of the four cardinal virtues, but nevertheless one important aspect of the virtue of righteousness. It is so important that it requires that man eventually "turns against himself" (zifan 自反), in order to fulfill the imperative of kindheartedness, propriety, and loyalty (zhong 忠).

Critique towards other schools. The book Mengzi is famous for the disputing force with which Meng Ke attacks his opponents, especially representants of the school of the Divine Husbandman (Shen Nong 神农) that argued that everybody should engage in agriculture (bing geng 并耕) in order to achieve an egalitarian society. He criticized the libertinist Yang Zhu杨朱 for his egoism (wei wo 为我 "for me"), and the Mohists for their egalitarian approach of universal love (jian ai 兼爱). Meng Ke likes to use parables to clarify his theories and to express simple, but crucial circumstances by analogies, like the people that is yearning for a good ruler like desiccated land for rain, or somebody who is looking at the point of a small hair instead of at the large balk. Overhasty methods are described in the allegory of the peasant helping his shoots to grow by lifting them up, and unappropriate criticizing others is described in the parable of a deserter running fifty paces laughing about a deserter running away a hundred paces wide. A man who used to daily steal a hen from his neighbour, promised not to steal a hen but once a month in the future, instead of instantly ending his misbehaviour. Another story speaks of a husband playing a rich man at home, while begging for alms when outside. A lot of these parables are very popular in China and have lost nothing of their attractiveness even today. The Mengzi is not only a great collection of philosophical thought but also a very important book contributing to the development of prose literature in ancient China.