4 ideas
The classical Yu Gong named the Min River as the Yangtze's origin, and later scholars repeated it. Xu argued in his essay 'Investigating the Source of the Jiang' that the Jinsha is the true source because it is far longer and runs much farther into the western highlands.
Treat a canonical description as a claim to be checked on the ground, not as a fact that settles the question. Standing at the landform, measuring distances, heights and flows, and tracing a watercourse to its end show where a revered text records only the limits of its authors' travel. Errors in authoritative sources persist because readers copy the text instead of going to see.
In Guangxi, Guizhou and Yunnan, Xu explored more than a hundred limestone caves and recorded sinkholes, tower peaks, underground rivers and dripstone formations. He did not treat them as isolated curiosities. He described their shapes, dimensions and orientations and linked the forms to water moving through and under the rock, which gives one of the earliest detailed field records of karst terrain.
In 1636, aged about fifty, Xu set out on his longest journey toward Yunnan with the monk Jingwen. In 1637 bandits attacked their boat on the Xiang River, Jingwen was wounded, and he later died in Guangxi. Xu carried his companion's remains on to Jizu Mountain in Yunnan and kept exploring until illness in his legs left him unable to walk, and he was carried home around 1640, dying soon after.