Neo-Confucian thinkers presented their project as a recovery of the authentic teaching of Confucius and Mencius against a millennium of Buddhist and Daoist distraction, and Zhu Xi wrote polemics against Buddhism by name. The metaphysics they built to make that recovery rigorous tells a different story. Classical Confucianism, the Analects and Mencius, is almost entirely an ethics of right relationship and cultivated conduct; it has no developed account of cosmology, no systematic treatment of mind, and no technical vocabulary for what underlies appearances. Song dynasty thinkers needed exactly that vocabulary to answer challenges Buddhism had been posing for centuries, and they built it in conversation with the traditions they were arguing against.
Zhou Dunyi's Supreme Ultimate diagram borrowed the visual grammar of Daoist cosmological charts. The concept of li, principle, developed by the Cheng brothers and systematized by Zhu Xi as the pattern inherent in each thing yet identical across all things, echoes the Huayan Buddhist teaching that one principle is present whole in every particular, li shi wu ai, principle and phenomena interpenetrating without obstruction. Wang Yangming's later doctrine of liangzhi, an innate moral knowledge already complete in the mind and needing only to be uncovered, runs remarkably close to the Chan Buddhist teaching of an inherent buddha nature obscured rather than absent. None of this makes Neo-Confucianism a disguised Buddhism; its ethical center, filial duty and cultivating humaneness within ordinary relationships, remained squarely Confucian. But the metaphysical scaffolding that let it defend that ethics against sophisticated rivals was built, in no small part, from what those rivals had already built first.