Chinese empirical philosopher, considered by many to have been the greatest thinker of the Qing period (1644–1911/12).

Born to poor parents, Dai educated himself by reading borrowed books. Although he passed his preliminary civil service examinations, he never passed the highly stylized jinshi exam, which would have given him the power and prestige of official office. Because of his reputation as a scholar, the emperor invited him in 1773 to become a court compiler in the Imperial Manuscript Library. In this position Dai was able to come into contact with many rare and otherwise inaccessible books. When Dai failed the civil service exam for the sixth time, in 1775, the emperor finally made him a jinshi by special decree, and Dai became a member of the Imperial Academy. Altogether he wrote, edited, and collated about 50 works, dealing mainly with mathematics, philology, ancient geography, and the Confucian Classics.

The Qing dynasty witnessed a revolution in philosophy in which the abstract metaphysical speculation of the Song (Song dynasty) and Ming were rejected for a more concrete, disciplined kind of evidential learning called Hanxue. Dai attacked the dualism of the Song thinkers, who he believed had been misled by Buddhist and Daoist influences. The Song philosophers held that human beings have a lower, more physical nature ( qi) that is responsible for the passions and a more spiritual nature ( li) that sets a limit on the material nature. Against this dualism Dai posited a monistic system. He argued that li is the immanent structure in all things, even desires. Knowledge of li does not suddenly appear during meditation, as some of the Song philosophers believed. It is found only after an arduous search, using precise methods, whether in literary, historical, philological, or philosophic investigation.

Dai utilized these careful investigative methods in his own research. In mathematics, he wrote a short discourse on the logarithmic theories of the English mathematician John Napier (Napier, John) and edited a collection of seven ancient mathematical works, the last of which is his own collation. In philology, he wrote several books, including a classification of ancient pronunciation. In addition he collated the classic of the 6th century, Shuijingzhu (“Commentary on the Classic of Waterways”), a study of 137 waterways in ancient China.

Because the Song philosophy had the patronage of the bureaucracy, Dai's contributions were largely ignored in the years after his death. But because his stress on the need for close empirical investigation resembles the “scientific” and pragmatic approach of Western philosophy, his ideas began to be studied again in the 20th century. In 1924 the bicentennial of Dai's birth was celebrated in Beijing, and in 1936 the Chinese scholarly world paid tribute to him with the publication of a complete and authoritative edition of his works, Dai Dongyuan xiansheng quanji (“Collected Writings of Mr. Dai Dongyuan”).