成唯識論演祕 6.174

6.174

釋曰：恐執虛空離心、心所，名非心法；或恐彼執能依空無，所依心法亦復不有，由斯故說三界唯心，披此觀《疏》，疏可悟矣。

Explanation: For fear that they would cling to space as being apart from mind and mental factors, calling it "not a mental phenomenon"; or for fear that they would cling to the supported space being nonexistent, so that the supporting mental phenomena would also not exist—for these reasons it is said that the three realms are cittamātra. If one opens and reads the Commentary in light of this, the Commentary can be understood.