仁王經疏 1.316

1.316

釋曰：第三、釋法空也。謂於十地，皆有始生、住生、終生不可得。地地有三，故成三十。

Commentary: Third, explaining the emptiness of dharmas. This means that in the ten bhūmis, initial birth, abiding birth, and final birth are all ungraspable. Each bhūmi has three, thus making thirty.