Étienne Fortier-Dubois

the Song came to depend increasingly on taxes on trade and so systematically encouraged it.

The Ming Emperors believed that Song policy had produced a weak, divided polity and had undermined the stability of traditional Chinese society.

Then the Mongols invaded, and after that came the Ming empire. And the Ming were quite the opposite of the Song. They wanted tight, centralized control of everything. They literally controlled where you could travel, and they needed a report from every merchant on how much stock he held in his warehouse at regular intervals. This was a recipe for... See more

This massive economy led to rich trading ties with a large number of its neighbors - including Japan, Korea, and the Indian Ocean marketplace. But the most significant economic ties across the totality of the Song Dynasty would be those not of its own making. Rather, they would be it its competitor and rival states to the north - first the Liao... See more

So, that shifts under the Song policy to a policy of "hey maybe let's not tax the pants off of the peasant farmers, and instead try something craazy." That "crazy" idea is to tax trade - both internal & with foreign entities.