Huan Tan 桓譚 (c. 43 BC–28 AD) was a Chinese philosopher of the Han Dynasty (202 BC–220 AD) and short-lived interregnum of the Xin Dynasty (9–23 AD). Huan's mode of philosophical thought belonged to an Old Text realist tradition supported by other contemporaries such as the naturalist and mechanistic philosopher Wang Chong (27–c. 100), the latter who Rafe de Crespigny states was probably heavily influenced by Huan Tan. Huan was a close associate of the court astronomer and mathematician Liu Xin (died 23 AD) as well as the author and poet Yang Xiong (53 BC–18 AD), and worked as an official under the administrations of Emperor Ai of Han (r. 27–1 BC), Wang Mang (r. 9–23 AD), Emperor Gengshi of Han (r. 23–25 AD), and Emperor Guangwu of Han (r. 25–57 AD).
In addition to his many rhapsodies, essays, and memorials, Huan's major work was the Xinlun (新論), "New Discussions", which was admired by Emperor Guangwu, despite Huan Tan's besmirched reputation for having closely associated himself with the regime of the usurper Wang Mang. His Xinlun is also the earliest text to describe the trip hammer device powered by hydraulics (i.e. a waterwheel) in order to pound and decorticate grain.
Huan Tan is reported by Yu Yingshi (b. 1930) to uphold self-contradictory views on immortality. On one hand, he is quoted to say that "the way of immortals" is a fabrication of the lovers for the strange; on the other, however, he was reported to admit the practice as genuine and efficient. Possible explanation lies in the fact that Huanzi xinlun 桓子新論 is a later compilation which might have confused his own statements with the quotations of his opponents.
- Crespigny, 338.
- Needham, Volume 4, Part 2, 392.
- Yü, Ying-shih. «Life and Immortality in The Mind of Han China». Harvard Journal of Asiatic Studies, Vol. 25, (1964—1965), p. 109.
- Crespigny, Rafe de. (2007). A Biographical Dictionary of Later Han to the Three Kingdoms (23-220 AD). Leiden: Koninklijke Brill. ISBN 90-04-15605-4.
- Knechtges, David R. (2010). "Huan Tan 桓譚". In Knechtges, David R.; Chang, Taiping. Ancient and Early Medieval Chinese Literature: A Reference Guide, Part One. Leiden: Brill. pp. 390–96. ISBN 978-90-04-19127-3.
- Needham, Joseph (1986). Science and Civilization in China: Volume 4, Physics and Physical Technology, Part 2, Mechanical Engineering. Cambridge: Cambridge University Press.
- Huan Tan in: Loewe, Michael, A Biographical Dictionary of the Qin, Former Han and Xin Periods (221 BC - AD 24), Leiden (Brill) 2000, ISBN 90-04-10364-3, pp. 164–165.
- "Hsin-Lun (New Treatise) and Other Writings by Huan T'an(43 B.C. - 28 A.D.)" Timoteus Pokora Michigan Papers in Chinese Studies NO.20 1975 Ann Abour Center for Chinese Studies The University of Michigan