成唯識論述記 3.216

3.216

述曰：其第三分前緣第二，却緣第四，第四前緣第三，何不通緣第二？非第二者，以無用故。設許得緣，涉重緣過，以無用故；或無緣彼之用，不說緣之，如自證分不緣於相，能緣各有分限可得故。然第三分得緣第四，現量等定，復不緣外，佛即不爾。

The Commentary states: The third part previously cognizes the second, and turns back to cognize the fourth; when the fourth previously cognizes the third, why does it not also cognize the second? It does not cognize the second because that would be useless. Even if it were granted that it could cognize it, there would be the fault of redundant cognition, because it would serve no purpose; or, having no function of cognizing it, it is not said to cognize it, just as the svasaṃvittibhāga does not cognize the nimittabhāga, because each cognitive capacity has its own ascertainable limit. But the third part can cognize the fourth, being fixed as direct perception and so forth, and does not again cognize the external; for a Buddha, however, it is not so.