大毘盧遮那成佛經疏 2.52

2.52

第廿三「云何河心？謂順修依因二邊法」者，此心性雙依二邊，或時修常或時修斷、或復邪正兼信，如河水雙依兩岸，其所漂流之物亦不定係一邊。此中對治，謂行人專心一境則能有所至到。若心不定守，能令事業俱辨，無此理也。

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.