大毘盧遮那成佛經疏 2.91

2.91

The sūtra says: 'Incomprehensible to all non-Buddhists.' In this tradition, two kinds of non-Buddhists are explained: external non-Buddhists, who, like someone seeing a clear pool, reactively give rise to fear and dare not approach it; and internal non-Buddhists, who, although able to swim in it, relieve the heat, remove dirt, and attain cool pleasure, yet do not realize that within it lies an immeasurable jewel king. In the first case, not entering, they do not recognize it; in the second case, having entered, they do not recognize it. Therefore it says: 'Incomprehensible to all non-Buddhists.'