There are many theories regarding this, such as there is a mystery related to Joe's birth, or being priest gained him a priority to take part in the mission to Tang, however the argument still has not met a settlement.
これに関しては定恵の出生に関わる謎がある、或いは僧侶になった方が唐留学に優位であった、等の意見があるが、未だ定説を見ない。