income as well as industrial and agricultural output. Some scholars have termed the phenomenon China's "medieval urban revolution". The Mongol conquest inflicted a large population loss and devastated the economy but the succeeding Ming dynasty brought a recovery in per capita incomes and economic output, surpassing Song dynasty heights. Late Ming laissez-faire policies such as nonintervention in markets and low taxes further stimulated commercialisation, as market agriculture replaced subsistence farming. Wage labour became increasingly common, as large-scale private industry developed, displacing indentured labor and often buying out government workshops. Historian Robert Allen estimates that family incomes and labor productivity of the Ming-era Yangtze Delta Region, the richest province of China, was far higher than contemporary Europe and exceeded the later Qing dynasty. Some contend that economic and social developments during the late Ming paralleled the development of Europe in the 18th and the 19th centuries and that China would have entered a modern age had there been no Manchu conquest and no Qing dynasty. The Ming regime was ideologically rigid but cities and new wealth allowed room for intellectual fervor and liberalization. New thinkers like Wang Yangming and Li Zhi challenged orthodox Confucianism and argued that the words of Confucius and Mencius were fallible and that wisdom was universal. They also questioned government power over the economy and personal rights. Scholars of the Donglin school protested increases in government taxation during the Wanli Emperor, and restrictions on freedom of speech, advocating a program similar to classical liberalism. Ming dynasty scholars also investigated western science, such as Archimedes. Evidence Supporters of the theory hold that the policies of the Qing dynasty slowed China's economic and scientific advancement and allowed Western nations to surpass China. Specific Qing policies cited include suppression of creative thought, literary persecution, discouragement of foreign trade, repressive domestic policies, rigid neo-Confucian emphasis on ideology rather than practical knowledge, disrespect for business and commerce, destructive fiscal and tax policy, as well as the devastation of the initial conquest itself. Restrictions on foreign trade Supporters most often point to Qing restriction on foreign trade as evidence of the theory. During the Ming dynasty, considerable commerce existed between China, Japan and Western Europe, estimated by Joseph Needham at nearly 300 million taels of silver from 1578 to 1644 (for comparison, the total Ming state revenues were from 20 to 30 million taels). However, during the Qing dynasty, foreign trade was prohibited