Taking recourse to the Aristotelian theory of four causes and the distinction between substance and accidents, as well as to the concept of intellectual consciousness (lingjue 靈覺), Matteo Ricci (Li Madou 利瑪竇, 1552–1610) argued that the notions of taiji 太極 and li 理 as expounded by Zhu Xi 朱熹 (1130–1200) cannot serve as the origin of existence, as their metaphysical equivalent God, does in Catholic Christianity. Taiji and li do not have consciousness and are moreover merely “accidents.” Unlike “substance,” an “accident” is something that is not essential for something to be what it is. Tasan Chŏng Yagyong 茶山 丁若鏞 (1762–1836) follows Ricci in claiming that li is but an accident, and no more than the “formal cause” of each individual object. A “formal cause” is what something is or should be, as distinct from the “material cause” (that of which it is made), the “efficient cause” (that which gives it shape or motion), or the “final cause” (the goal for which it is intended). Ricci and Tasan did not acknowledge the role of li as the “principle, ground, cause or the reason for the existence and operation of qi 氣” (suoyi 所以) as held by Zhu Xi’s school of nature and principle. Like Ricci, Tasan also dismissed the concept of taiji as “the undifferentiated state before heaven and earth came to be divided.” However, Tasan diverged markedly from Ricci with respect to the critique of the tenet “Nature is in fact principle (Xing ji li 性卽理)”. Although Tasan and Ricci both separated xing 性 from li and deconstructed the meaning of xing from its connotation of original moral human nature, Tasan’s reconstructed xing is quite different from Ricci’s understanding “nothing other than the fundamental (ben 本) essence (ti 體) of each category of things.” Tasan’s unique contribution is his new definition of xing (K. sŏng) as the appetite or preference/proclivity [for the moral good] (K. kiho, C. shihao 嗜好), which is quite Mencian in its affirmation of the presence of an incipient tendency toward the good in human beings. That he mentions that this xing (K. sŏng), although divested of its a priori metaphysical connotations, is still mandated and bestowed by heaven as stated in the Zhongyong 中庸 [Doctrine of the Mean] further testifies to Tasan’s continued engagement with the classics.