Abstract: This article offers a new interpretation of Mencius's concept of the moral heart/mind, proposing that its inner or ontological structure consists of four interrelated and penetrating aspects: pity and compassion ("恻隐"), shame and aversion ("羞恶"), modesty and yielding ("辞让"), and a sense of right and wrong ("是非"). In this framework, duan("端") in siduan("四端") refers not only to the "beginning" or "sprout, " but also to a distinct "aspect" of the moral heart/mind. The siduan are thus not merely four examples of moral responses to different situations, but rather four constitutive dimensions of the moral heart/mind itself. From the perspective of moral psychology, compassion represents empathetic concern focused on the suffering of others; shame is a constructive moral emotion akin to guilt, but more significantly, it arises when one fails to show compassion; yielding is an expression of compassion in contexts where one can relinquish personal gain for the benefit of others; and a sense of right and wrong emerges from the interplay of compassion and shame. These four aspects serve both as moral emotions and as moral reasons. Within the moral mind, they interact through relationships of unification ("统摄") and mutual generation and restraint ("生 克"). This dynamic reflects the unity of reason and emotion, expressed in both manifest and latent forms ("显隐"). By analyzing this structure, the article aims to provide a more accurate and nuanced understanding of Mencius's moral philosophy and its development in the Song-Ming tradition. It also seeks to offer alternative perspectives for research in contemporary moral psychology.