The Korean Neo-Confucianism of Yi Toegye and Yi Yulgok (Part 145)
The question is, Why did Toegye maintain it as his favourite text? The Classic of the Mind-and-Heart, or Heart Classic for short, has almost no mention about the rational dimension of the Cheng-Chu teachings, namely the “investigation of things” and “extension of knowledge.” Rather, its main emphasis is focused on the moral and spiritual dimension of self-cultivation. For his reason, it became the most important text for Toegye, who quoted it frequently in discussing the Cheng-Chu simhak. what he liked about it is especially its preoccupation with an inner-directed type of moral self-cultivation, rather than a rational study of principles and things. Invoking Sung thinkers including Chen Te-hsiu as not merely as metaphysicians but more often as moralists, Toegye’s lifelong dedication to study Chen’s Heart Classic clearly demonstrates his ultimate concern for simhak. surely, it had a significant impact on Toegye’s philosophy and practice of self-cultivation that tended to move beyond traditional Cheng-Chu norms.
Cheng-Chu teachings on mind cultivation focuses on its dual concept of the mind in terms of the “human mind” (insim) and the “moral mind” (tosim). In the third article of his Yokcho so, for example, Toegye cites the so-called Sixteen-Character Transmission of the Mind-and-Heart, an obscure passage from the Book of History: “The human mind is precarious; the moral mind is subtle.