大毘盧遮那成佛經疏 2.52

2.52

Twenty-third: regarding "What is the river mind? It means according with and cultivating the dharma of relying on the two extremes": the nature of this mind is to rely simultaneously on two extremes—sometimes cultivating eternalism, sometimes cultivating annihilationism, or believing concurrently in both the heterodox and the orthodox, just as river water flows relying on both banks, and the objects carried along by it are also not fixed to one side. The antidote here is that if a practitioner focuses their mind on a single object, they can achieve their destination. If the mind does not maintain a fixed focus, expecting that undertakings can all be accomplished is impossible.