仁王護國般若波羅蜜多經疏 2.127

2.127

Explanation: As for the "six sense spheres", they are the spheres of the eye, ear, nose, tongue, body, and mind. The first five are form; as the Abhidharmakośa-bhāṣya says, the body indriya has nine substances, and the remaining indriyas have ten substances. The sixth, the mind sphere, is a mental dharma, because they are all the gateways through which mind and mental factors arise. The real mark is not them, hence it is said to "not abide"; its substance pervades the six sense spheres, hence it is said to "not be separated". From here is the sixth, where three pairs are explained together.