俱舍論疏 3.37

3.37

「論曰至是有情本」，釋所依也。內六處是有情本故是心所依，外六處非有情本故非心所依。有情本者，內六處中全是有情，外六處中一分是情，由內六處故，有扶根四境等故，根是其本。

"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.