大毘盧遮那成佛經疏 2.91

2.91

經云「一切外道所不能知」者，此宗中說有兩種外道：外外道，猶如覩見清潭，逆生怖畏，不敢習近；內外道，雖能游泳其中，適熱除垢得清涼樂，然不覺是中有無量寶王。一則不入而不識，二則入而不識，故云一切外道不能知也。

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.'