大乘起信論義記 3.383

3.383

問中二：初陳疑，謂有虛空處皆有世界，有世界處皆有眾生，有眾生處皆有心行，如是境界分齊難知也。若無明下，正設難。難云：非直外境無邊分齊難知，亦復內盡心想，云何得了也？

The question has two parts: first, stating the doubt: wherever there is space there are worlds, wherever there are worlds there are sentient beings, and wherever there are sentient beings there are mental activities; such boundaries of spheres are difficult to know. From 'If avidyā' onward, directly raising the objection. The objection says: not only are external spheres limitless and their boundaries difficult to know, but internally mental thoughts are completely exhausted; how can they be fully understood?