This paper examines one of the most important works in Korea’s intellectual history, by one of its greatest scholars. T’oegye 退溪 Yi Hwang 李滉 (1501-1570), synthesised the Song dynasty 宋朝 metaphysics of Zhu Xi 朱熹 (1130-1200) in his magnum opus, Seonghak sipto 聖學十圖 (The Ten Diagrams on Sage Learning), reflecting the maturity of Korea’s Neo-Confucian tradition, and entirely shaping its future trajectory on the Korean peninsula, while also influencing the development of Neo-Confucian thought in Japan. I will delineate the scope of The Ten Diagrams on Sage Learning, while assessing T’oegye’s ‘Humanistic’ guiding discourse, which attempts to develop one’s sense of Humanity (仁, K. in), in the Confucian sense, through a rigorous process of self-cultivation, one of the salient features of East Asian thought. T’oegye elucidates our place in the ‘moral’ universe as the highest form of Principle, heightening our sense of responsibility towards others, emphasised in his diagrams on learning, which also calls on us to put that learning into actual practice in our daily lives. The text highlights the special role of the king who is expected to be a moral exemplar to his people, representing the essence of ‘sagehood’. The diagrams also represent T’oegye’s sophisticated meta-psychological analysis of the heart-and-mind (心K. sim) aimed at developing the psychological character of a ‘sage king’, and ultimately leading him to cultivate an impartial mind and attitude – the completion of a man of the Confucian Way – epitomising Humanity for his people.