Vigilance in solitude (shendu 慎獨) is a core concept in Confucian theories of self-cultivation practice (gongfu 工夫). Situated within the intellectual contexts of Song–Ming Neo-Confucianism and the Ming–Qing transition, this study takes as its central question how morality is possible within the domain of solitary knowing (duzhi 獨知). It traces the genealogy of interpretations of shendu across three canonical loci: the chapter on making the intentions sincere (chengyi 誠意) in the Daxue 大學 (Great Learning), the opening section of the Zhongyong 中庸 (Doctrine of the Mean), and the "Liqi" 禮器 (Ritual Vessels) chapter of the Liji 禮記 (Record of Rites). Particular attention is given to the readings of Confucian thinkers of the Ming–Qing transition such as Liu Zongzhou 劉宗周 (1578–1645), Huang Zongxi 黃宗羲 (1610–1695), Sun Qifeng 孫奇逢 (1584–1675), Li Yong 李顒 (1627–1705), and Wang Fuzhi 王夫之 (1619–1692). It also considers reflections rooted in classical scholarship (jingxue 經學) by Gu Yanwu 顧炎武 (1613–1682), Chen Que 陳確 (1604–1677), Mao Qiling 毛奇齡 (1623–1716), and Yan Ruoqu 閻若璩 (1636–1704). By comparing these readings with those of Song–Ming Neo-Confucians such as Zhu Xi 朱熹 (1130–1200) and Wang Yangming 王陽明 (1472–1529), the study explores the continuities and ruptures that marked the transition from Song–Ming Neo-Confucianism to Ming–Qing thought. Rather than surveying thinkers one by one, as is conventional, it proceeds through five themes. The first is the genesis of evil within the domain of solitary knowing, where vigilance in solitude meets self-deception (ziqi 自欺). The second is the temporality of vigilance, framed by the notion of incipience (ji 幾). The third is the delimitation of the practicing subject in terms of oneness (yi 一) and solitude (du 獨). The fourth is vigilance in solitude as everyday ritual practice. The fifth is its transcendent dimension of being watched over (jianlin 鑒臨), together with its place in the interactions among the Three Teachings (sanjiao 三教). The study shows that thinking on shendu diverged significantly during the Ming–Qing transition. This shift took plural forms: the resituation of shendu as a problem of classical exegesis, an externalization of cultivation through ritual learning (lixue 禮學), and the integration of gongfu into daily life. The transformation responded to the spiritual predicament of Ming loyalists (yimin 遺民) after the fall of the Ming. It also offers resources for rethinking moral self-examination, immanent transcendence, and moral autonomy, resources that can be brought into dialogue with Western ethical traditions.