俱舍論疏 3.37

3.37

"The Śāstra states: ... are the root of sentient beings" explains the basis. The six internal āyatanas are the root of sentient beings, and therefore are the basis of the mind; the six external āyatanas are not the root of sentient beings, and therefore are not the basis of the mind. As for "the root of sentient beings," the six internal āyatanas are entirely sentient beings, whereas of the six external āyatanas only a portion is sentient. It is due to the six internal āyatanas that the four sense spheres supporting the faculties exist; therefore, the faculties are their root.