Living about two centuries after Confucius, Mengzi (372–289 BCE; known in the West as Mencius) is the most important thinker to deepen his ethical teaching and give it a philosophical foundation. Confucius had described how virtuous conduct should be; Mengzi answered a more fundamental question: why can a human be virtuous? What, in fact, is human nature?

His answer was optimistic and, for its time, contested: human nature is good at its core. Mengzi defends this with a famous thought experiment: anyone who sees a child about to fall into a well feels, suddenly and without any calculation of gain, a pang of alarm and compassion. This sudden feeling is proof of a "seed of compassion" innate in the human. For Mengzi there are four such moral seeds in a person: compassion, shame, deference, and the sense of right and wrong. These can grow, respectively, into the four great virtues — humaneness (ren), righteousness, propriety, and wisdom.

But a seed does not become a tree on its own. Mengzi explains the role of education and effort with an agricultural analogy: innate goodness is a sprout that must be watered and tended; neglected, it withers; under bad conditions, it is corrupted. Moral development is not the imposition of an external mould but the nourishing of this natural tendency within.

Mengzi also gave rulers a bold political counsel: a sovereign's legitimacy rests on the "Mandate of Heaven" (Tianming), but Heaven sees through the eyes of the people; a ruler who oppresses his people and neglects their welfare loses this mandate and deserves to be overthrown. This is an early thought emphasising the responsibility of power toward the people. Mengzi's thesis that "human nature is good," together with Xunzi's counter-thesis that "human nature is bad," formed the fundamental debate of Chinese philosophy for centuries.