This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.
Zhou Dunyi's Tongshu, the Penetrating the Book of Changes, extends the cosmology of his Taijitu Shuo into an ethics of sincerity, a documented second treatise distinct from the Taijitu Shuo's own diagram and its account of how the Supreme Ultimate generates yin, yang, the Five Phases and the ten thousand things. Where that diagram explained how a moral cosmos comes to exist at all, the Tongshu addressed a different question, how a person is meant to live inside the cosmos the diagram had already described, a documented shift from cosmology to ethics distinct from a second work that simply restated the first in different words.
Sincerity, in the Tongshu's own account, names the human alignment with the cosmic pattern the Taijitu Shuo had set out, a documented ethical concept distinct from a virtue defined without reference to that larger cosmological picture. The two treatises together gave later Neo-Confucians both a metaphysics and a practice, a documented pairing distinct from a philosophy that offered an account of the cosmos with no corresponding account of how a person should act within it.
That Zhou Dunyi followed his own diagram of how the cosmos generates a moral order with a second, separate treatise on how sincerity lets a person live inside that same order, rather than leaving the practical question to later interpreters, marks the Tongshu's place in his work as a documented case where a philosopher who explained the cosmos also insisted on explaining, in a text of its own, what a human being owed it in return.