大方廣圓覺修多羅了義經略疏 2.611

2.611

六、獨足雙頭觀。白澤圖中有山精，頭如鼓，有兩面，前後俱見，此喻靜幻雙照，二利齊運，如雙頭也。單寂觀如獨足也。

Sixth, the contemplation of the single leg and double head. In the Baize Tu there is a mountain spirit whose head is like a drum, having two faces that see both forward and backward; this illustrates doubly illumining stillness and illusion, putting both benefits into operation simultaneously, like the double head. The solitary quiescence contemplation is like the single leg.