法華論疏 3.314

3.314

上明四種數：一世界不可知；二末世界為塵不可知；三約塵數劫，劫不可知；四明佛壽復過上三數不可知。今是第五數，復過前四，以壽數不可知，所以為常。

Above, four kinds of numbers were explained: first, the worlds are unknowable; second, grinding the worlds into dust is unknowable; third, based on the dust-count kalpas, the kalpas are unknowable; fourth, explaining that the Buddha's lifespan exceeds the above three numbers and is unknowable. Now this is the fifth number, which again exceeds the previous four; because the number of lifespan is unknowable, it is therefore permanent.