He hypothesized that after Zheng's sixth voyage, the Chinese must have produced a world map, but it was probably destroyed when the emperor adopted an isolationist policy and sealed the country off from the outside world.
他假設鄭和第六次下西洋後，一定繪製出過一張世界地圖，而這張地圖在明成祖駕崩後的鎖國政策裡遭到銷毀。